When Montgomery County state’s attorneys set out to prove David Rich Hang killed his estranged stepdaughter in her Gaithersburg home May 31, 2011, one of the first pieces of information they secured for trial were records from cell phone towers used by Hang’s phone that day.
And while the records presented by prosecutors seemed to place Hang near the scene of the crime — 745 Raven Ave. — at key times that afternoon, county public defenders were able to create enough doubt in the minds of jurors to keep their case alive. The jury eventually convicted Hang of the first-degree murder of 12-year-old Jessica Nguyen, but the most damning evidence in his case came in the form of DNA data retrieved from the sheath of a small sword found at the scene, not cell tower data.
Requesting records and cell phone data has become a standard procedure for attorneys and police in most major criminal investigations, but experts are quick to point out that the records are kept for billing purposes, not locational data, making them much less precise than GPS tracking for determining a suspect’s exact location. Nonetheless, cell tower data can be useful.
When a user places a call on a cell phone, service for that call is provided by the tower with the strongest signal in the immediate area, said Chris Taylor, a telecommunications expert for Virginia-based CyTech Services. Even so, the nearest tower isn’t always the closest to the caller, he said.
“Your phone is tracking up to 12 cell towers at any given time, and whichever tower is providing the strongest signal, your phone is going to say, ‘I want that tower to provide the services for this call,’” he said, explaining that a single call can be “handed off” to several different towers between the time the call is made and when it ends, all of which are logged in tower records.
In ideal conditions with level ground and no interference from tall buildings or the like, the average cell tower can serve a 20-mile radius, Taylor said. In a more densely populated area like much of Montgomery County, cell towers often are bunched closer together to ensure reliable service between buildings and other obstructions, he added.
“It’s still relatively accurate; we did a case in Montgomery County a little while back where we were able to narrow it down to a single apartment building where the call was being placed from,” he said.
In spite of this, using cell tower records for locational data often is seen as a double-edged sword by attorneys, said Montgomery County Public Defender Brian Shefferman.
“When I’m using them to prove my client’s innocence and they do tend to prove my client’s innocence, then they’re very reliable, but when the prosecution is trying to use them to prove my client’s guilt, then obviously they’re totally unreliable,” Shefferman said with a laugh.
While he has subpoenaed cell tower records frequently in the past, Shefferman attempted to block prosecutors from entering similar data at Hang’s trial. Shefferman has seen several cases overturned by cell tower data, but he believes that, given their lack of precision, such data is perhaps best used as a general indicator and not the basis of an entire case.
“If somebody says, ‘Look, I was in California when this thing happened,’ and it turns out there’s tons of phone calls from the state of Maryland, then, okay, that’s one thing, but when you’re trying to get extremely precise locations, frankly, I think it’s really unclear how reliable that is,” he said.
Police relied on cell tower records for a similar purpose just prior to the Oct. 13, 2011, arrest of Curtis Maurice Lopez, who originally told police he was in New Jersey when they called to interview him regarding the killing of his estranged wife and 11-year-old stepson in Germantown. Cell tower records obtained that same day indicated Lopez’s phone was “pinging” off towers in North Carolina, where he was arrested, police said.
County State’s Attorney John McCarthy agreed with Shefferman that tower records are best used to support an attorney’s version of the facts. While DNA evidence played a bigger role in Hang’s conviction, the cell tower records helped “poke holes” in the defense’s statements.
As in any circumstance where technical data is being presented to a jury, attorneys also must take care to provide experts who can fluently explain the value — and limitations — of the data, McCarthy said.
“They know and recognize the stuff is coming, they have people who are skilled and can explain the information,” he said. “It's become standard operating procedure for them as well as for us.”
Audrey Chang, an AT&T spokeswoman, declined to provide details regarding the company’s policy on releasing records, but AT&T does require a subpoena from court or local law enforcement before records are provided, she said.