Usually, when a user places a call, the cell phone connects to the site with the strongest signal. But a key feature of the network is to provide overlap in coverage to avoid disconnection or dropped calls. Under this design, one cell site will pick up a call when another goes down; this process is known as a “handoff.” For that reason, handing-off will occur as a cell phone user moves through different coverage areas. But the geographic location of the user is not the only reason for handing off calls because other factors may affect the signal strength between a cell phone and site.
The federal courts have tended to allow historical cell site analysis to be admitted in the form of expert testimony in criminal trials. That analysis uses cell phone records and cell tower locations to determine, within some range of error, a cell phone’s location at a particular time. United States v. Hill, 818 F.3d 289, 295 (7th Cir. 2016). In the Hill case, the parties did not dispute that historical cell site analysis is a proper subject of expert testimony. Id. at 296. The expert in the Hill case was not a telecommunications engineer or an employee of the cell phone provider; the expert was a special agent of the Federal Bureau of Investigation (FBI). Id. Oddly, the court of appeals found that his testimony was “somewhat helpful to the trier of fact—even if not that helpful.” Id. at 298.
In a more recent decision from that court, the same FBI agent was allowed to give his expert views arising from historical cell site analysis. United States v. Adame, 2016 WL 3536655, at *3 (7th Cir. June 28, 2016). In part, the agent testified that his cell site analysis could determine the approximate area where a phone would have to have been when it placed or received the calls in the records. Id. Moreover, the agent testified that an FBI software package allowed him to determine which cell tower a phone was using. Id. In a stroke of good fortune for the prosecution, the defense did not raise any foundational objections to the reliability of that software at trial. Id. at *3 n.1. The court of appeals went out of its way to affirm the admission of the agent’s expert testimony before the jury. Even if the agent’s testimony about the historical cell site analysis was inadmissible, any error was viewed as “harmless.” Id. at *6. According to the court, all of the agent’s expert testimony was cumulative because it was corroborated by other witnesses at trial. Id.
In both of these decisions by the Seventh Circuit Court of Appeals, the FBI agent who gave the expert testimony was careful to make disclaimers to the jury. In the Hill case, the agent emphasized that the defendant’s cell phone’s use of a cell site did not mean that the defendant was right at that tower or at any particular spot at that tower. Hill, 818 F. 3d at 298. The court declared: “This disclaimer saves his testimony.” Id. In the Adame case, the agent qualified his testimony again, with the approval of the same appellate court. In particular, the agent acknowledged to the jury that he was unable to tell from his historical cell site analysis whether the defendant was at a specific address at any point during the evening of the crime. Adame, 2016 WL 3536655, at *6.
Is historical cell site analysis a junk science? Some observers think so. Professor Edward Imwinkelried of the University of California Davis Law School has been very critical of this cell site analysis. In a recent interview, Professor Imwinkelried opined that the methodology being employed by the expert doesn’t support the inference that the expert draws about the caller’s location. “Interview with Professor Edward J. Imwinkelried,” Litigation, Vol. 42, No. 4 (Summer 2016), at 25. Another opponent of historical cell site analysis is Michael Cherry, chief executive officer of a Virginia-based consulting firm. Mr. Cherry has called it a junk science that should never be admitted in any court for any reason. M. Hansen, “Prosecutor’s Use of Mobile Phone Tracking Is ‘Junk Science,’ Critics Say,” A.B.A. J., June 1, 2013. He cannot believe that such an easily disproved technique is still routinely used in the courts.
It appears that expert testimony based on historical cell site analysis is here to stay, at least in the foreseeable future. But it must be qualified by a string of disclaimers. At some point, one might ask whether the disclaimers will serve only to confuse the jury or, worse, to mislead them. It will be no surprise to see more challenges to the admission of such expert testimony under Federal Rule of Evidence 403. Where the historical cell site analysis is uncorroborated by witness testimony, it could be highly vulnerable to attack by a defendant’s trial counsel.
Keywords: litigation, expert witnesses, criminal law, cell phones, expert testimony
Alexander S. Vesselinovitch is a partner at Freeborn & Peters, LLP, in Chicago, Illinois.