Imagine this situation: The government has charged a defendant with Hobbs Act robbery. The indictment alleges that the defendant entered a convenience store at 1 a.m., pointed a gun at the clerk, and threatened to shoot him if he did not hand over the contents of the cash register. The clerk did as he was told, and the defendant fled. Security camera video shows a tall, stocky man dressed in dark clothing, with a ski mask covering all of his face except for his eyes. The clerk is unable to identify the assailant.
The Federal Bureau of Investigation (FBI) gets the local news station to run a still from the video on the evening news and asks that anyone who can identify the robber contact the agency. Someone does, anonymously, telling them that the robber was someone known by the street name of “Bad Eddie” in the neighborhood where the store is located. Further investigation reveals that the real name of Bad Eddie is Edward Smith and that he has a cell phone subscription with a cell phone provider called Horizon. Armed with this information, the FBI obtains a court order directing Horizon to produce the billing records for Smith’s cell phone for the days before, during, and after the robbery. By analyzing these records, a special agent from the FBI’s Cellular Analysis Survey Team (CAST) issues a report with accompanying maps that identify the cell towers that Smith’s cell phone connected to at various times before and after the robbery. One call occurred at 12:55, just five minutes before the robbery, and it appears on the map at the same location as the convenience store. Another call, an hour after the robbery, appears in the same location as the apartment where Smith lives.
Smith’s lawyer files a pretrial motion in limine challenging the admissibility of the testimony of the FBI CAST special agent as to her analysis of the historical cell phone data for Smith’s phone. The essence of the challenge is that the methodology used to analyze the location of Smith’s phone has not been shown to be reliable and that the maps are excessively prejudicial because they appear to pinpoint the exact location of the cell phone immediately before and after the robbery.
The government files a response, arguing that federal courts routinely admit this type of evidence and that its general acceptance by the law enforcement community meets the requirements of Federal Rule of Evidence 702. Alternatively, the government contends that the CAST special agent’s testimony on how cell phones communicate with cell towers and how she plotted the cell tower locations on the map is factual in nature, not opinion testimony, which the agent can testify to under Rule 701 because it is based on her personal knowledge, so Rule 702 is not even implicated at all. How should the judge rule?
This one’s easy, right? Historical cell phone record analysis routinely is admitted in state and federal courts to show the location of a cell phone at a particular place and time. This is a “no-brainer” for the judge: deny the motion; the evidence comes in, correct? Not so fast. It certainly is true that almost every federal trial court faced with this issue routinely has admitted historical cell phone location analysis evidence without much analysis. United States v. Hill, 818 F. 3d 289, 297 (7th Cir. 2016) (“District courts that have been called upon to decide whether to admit historical cell-site analysis have almost universally done so.”). And the fact that so many have done so is likely to influence the thinking of a district judge asked to undertake a “Daubert challenge” to this kind of evidence. It also is true that some circuit courts have brushed aside challenges to the reliability of the methodology used in historical cell-site location analysis because it has been universally accepted as reliable within the law enforcement community. United States v. Schaffer, 439 F. App’x 344, 347 (5th Cir. 2011) (stating that historical cell site evidence “is neither untested nor unestablished” because the FBI successfully had used it more than 1,000 times and many courts had admitted it).
No Authoritative Ruling
But what may be surprising is that despite the frequency with which historical cell-site location evidence is admitted, “[n]o federal court of appeals has yet said authoritatively that historical cell-site analysis is admissible to prove location of a cell phone user.” Hill, 818 F.3d at 297. Further, the circuit courts have not been of one mind when it comes to whether evidence about how cell phones connect to cell towers and how this information affects the ability to determine the location of a cell phone user involves expert testimony or may be accomplished through lay testimony. Compare, e.g., United States v. Graham, 796 F.3d 332, 364–65 (4th Cir. 2015) (concluding that records custodian for cell phone service provider could testify as a lay witness about how cell towers communicate with cell phones; FBI agent could testify as lay witness about using mapping software to “plot locations of certain cell sites [accessed by defendant’s phone] . . . listed in . . . [cell phone company’s billing] records, the business establishments robbed, and . . . [defendant]’s apartment,” including the “dates and times of inbound and outbound calls”), reversed in part on other grounds on reh’g en banc, 824 F.3d 421 (4th Cir. 2016), with United States v. Natal, 849 F. 3d 530, 533 (2d Cir. 2017) (“[W]e hold that testimony on how cell phone towers operate constitutes expert testimony and may not be introduced through a lay witness.”). In the absence of a clear majority rule on this issue, it would be wise to heed this advice:
We need not hold that all evidence related to cell phone towers necessarily requires expertise. But we caution that the line between testimony on how cell phone towers operate, which must be offered by an expert witness, and any other testimony on cell phone towers, will frequently be difficult to draw, and so both litigants and district courts would be well advised to consider seriously the potential need for expert testimony.
Natal, 849 F.3d at 536 (emphasis in original).
It also is worth noting that, while historical cell-site location evidence has been allowed into evidence routinely in criminal trials, some courts have expressed skepticism regarding aspects of its reliability. Take, for instance, United States v. Evans, 892 F. Supp. 2d 949, 956–57 (N.D. Ill. 2012). There, the district court found that an FBI special agent was qualified (despite not being an engineer or having worked for a cellular network provider) to testify about how cellular networks operate and that a phone registered to the defendant connected to various cell towers during the course of a criminal conspiracy, because this information could aid the jury in determining the location of the cell phone at a particular time. Id. at 956. But the court excluded testimony that the agent had used a theory described as “granulization” to “estimate the range of certain cell sites based on a tower’s location to other towers,” which, in turn, allowed him “to predict the coverage overlap of two closely positioned towers . . . with a zero percent error rate.” Id.
Similarly, the Sixth Circuit was pointed in its criticism of the rationale used by some courts to conclude that historical cell tower records analysis was admissible to prove the location of a cell phone. See United States v. Reynolds, 626 F. App’x 610 (6th Cir. 2015). As for the argument that historical cell site analysis was reliable because so many courts have admitted it into evidence, the court noted that ‘“judges are not scientists and do not have the scientific training that can facilitate the making of [scientific] decisions’. . . . For this reason Daubert identified the ‘scientific community,’ rather than federal courts, as the relevant group in which acceptance is an indicator of a technique’s reliability.” Id. at 616 (internal citations omitted). As for the argument that historical cell tower records analysis universally has been accepted as reliable by law enforcement organizations because it had been used successfully over 1,000 times, it noted:
This claim appears to be precisely the sort of “ipse dixit of the expert” testimony that should raise a gatekeeper’s suspicion. While being successfully employed “1000 times” may sound impressive, the claim is not subject to independent peer review and fails to establish an error rate with which to assess reliability because there was no information on how many times the technique was employed unsuccessfully.
Id. (emphasis in original, internal citation omitted).
In a recent decision, the Seventh Circuit concluded that certain aspects of historical cell site analysis may meet the requirements of Daubert and Rule 702:
Historical cell-site analysis can show with sufficient reliability that a phone was in a general area, especially in a well-populated one. It shows the cell sites with which the person’s cell phone connected, and the science is well understood. . . . A mathematical error rate has not been calculated, but the technique has been subject to publication and peer criticism, if not [exactly] peer review.
Hill, 818 F.3d at 298.
But it was quick to add that other aspects of historical cell site analysis may be problematic:
Our concern is that the jury may overestimate the quality of the information provided by this analysis. We therefore caution the government not to present historical cell-site evidence without clearly indicating the level of precision—or imprecision—with which that particular evidence pinpoints a person’s location at a given time. The admission of historical cell-site evidence that overpromises on the technique’s precision—or fails to account adequately for its potential flaws—may well be an abuse of discretion.
Id. at 299.
As one court explained,
[t]he reason for this cautionary approach lies with the characteristics of how cell phones communicate with the cell towers of a cellular network, and the many factors that can influence whether the cell site [a cell phone] connected to (the one with the strongest signal) is actually the one closest to the phone at the time that it connected.
United States v. Medley, No. PWG-17-242, slip op. at 12 (D. Md. May 9, 2018).
Many factors can influence the strength of the signal between a cell phone and a cell tower:
First, the technical characteristics of cell sites may affect signal strength: (1) the number of sites available; (2) maintenance or repairs being performed; (3) the height of the cell tower; (4) height above sea level; (5) wattage output; and (6) range of coverage. Second, technical characteristics of the antennas on the cellular sites may affect signal strength such as the number of antennas, the angle and direction the antenna is facing, height of each antenna, and call traffic processed through each antenna. Third, technical characteristics of the phone, such as the wattage output and generation of the phone’s broadband capacity, may affect signal strength. Fourth, signal strength may depend upon environmental and geographical factors including the weather, topography, and level of urban development. Finally, indoor or outdoor use of the phone may alter the strength of the signal.
Aaron Blank, The Limitations and Admissibility of Using Historical Cellular Site Data to Track the Location of a Cellular Phone, 18 Rich. J. L. & Tech. 3, 7 (2011); see also Matthew Tart et al., Historical Cell Site Analysis—Overview of Principles and Survey Methodologies, 8 Digital Investigation 185, 186 (2012) (“The service area of a given cell is dependent on many factors including the height of the antenna (aerial), the power used, the location of other cells and the geography of the land (hills, trees, etc.) including surrounding buildings.”).
It also is noteworthy that there are degrees of accuracy within cell-site location methodology. The most accurate method is known as “triangulation,” which works this way:
A cell phone’s signal will often be received simultaneously by more than one cell site when operating in areas with high concentration of cell sites and overlaps in coverage. When this occurs, a mathematical process called triangulation may determine the phone’s location if either: (1) three points receiving the signal are known; or (2) two points receiving the signal are known, along with the direction in which the cell site received the signal. The accuracy of triangulation varies depending on a number of factors such as the density of cell sites. Urban areas tend to have a higher density of cell sites; therefore triangulation is most feasible in those areas.
Blank, supra, at 11.
But there are shortcomings:
Often historical cell site records only indicate the date, time, and duration of calls, whether calls are inbound or outbound, and show the originating and terminating cell sites for calls received or placed on the phone. Accordingly, triangulation cannot determine the location of the phone because either the phone connected with only one site (i.e., the originating and terminating cell sites are the same) or only two sites are known at different times (i.e., at the beginning and end of the call) without directional information. . . . If triangulation is not possible from the available records, then these records only show, at most, the phone’s coverage areas at the beginning and end of the call.
Blank, supra, at 13.
So, what are the takeaways from all of this? First, historical cell-site location analysis inherently involves technical or specialized knowledge, so trying to introduce it without a sponsoring expert witness is risky business. And if the court concludes (as most have) that it does involve expert testimony, then the party that intends to introduce the historical cell site evidence had better have made its required disclosures under Federal Rule of Criminal Procedure 16 or Federal Rule of Civil Procedure 26(a)(2) in a timely fashion.
Second, the trend in the court cases that have given thoughtful consideration to this evidence is to recognize that its admissibility is contingent on the proponent accurately disclosing the limits of its reliability and the candor of the sponsoring witness in acknowledging these limits. As the Seventh Circuit pointed out in United States v. Hill, it may be abuse of discretion for a trial court to admit historical cell site evidence that overpromises its reliability. In many instances, the best that it can establish is that the cell phone was located somewhere within the geographical space of a particular cell sector, which may be quite large in rural areas but relatively smaller in urban ones.
Third, there are many factors that can influence the strength of the radio wave connection between a cell phone and a particular cell tower, and the tower with the strongest signal may not be the one closest to the phone. And there are various degrees of accuracy of historical cell site analysis depending, for example, on whether or not the records permit triangulation. Attorneys need to know about these factors so they can properly prepare their witnesses for direct examination or confront them on the limitations of the methodology on cross.
Finally, judges should be cautious about admitting this evidence in the face of an evidentiary challenge just because so many courts in the past have done so, often without any real consideration of whether it meets the requirements of Rule 702.