Law enforcement officials have used cellular tower data as an investigative tool for more than a decade.
On Tuesday, Tampa officers used it to link Howell Donaldson III to the Seminole Heights homicides.
According to Donaldson's arrest report, police got call detail records for cell towers near the crime scenes. They showed that within minutes of the murders, Donaldson's phone was "geographically associated" with those towers.
John Sawicki, a Tallahassee-based computer forensics analyst who specializes in cellphone technology, said investigators likely used the data like this:
Police identified towers that serviced the locations of the homicide scenes and requested the records from AT&T. Then they'll do what's called a "tower dump," where they'll comb through phone numbers that have been serviced by that tower to find a common number.
If the same phone number is found at all locations at the right times, Sawicki said, "the chance of finding your own actual suspect really increases significantly."
Records show that Donaldson also allowed police to look at his phone. Cellphones can log location data unless users switch off the GPS function by checking the "privacy'' or "location'' settings.
Police said they found that Donaldson was near the crime scenes on Oct. 9, Oct. 11 and Oct. 19.
"The significance of the dates and times are that they correspond with the dates and times of the first three murders," the report reads.
Jerry Theophilopoulos, a Pinellas criminal defense lawyer, said tower data is the kind of evidence that a jury easily grasps.
"Juries like things that aren't gray. They like it black and white," he said. "If you've got him in the area, at least his cellphone in the area, that's going to be some powerful evidence against him."
Clearwater defense lawyer Stephen Romine said there are variables: What geographic range does the tower cover? How many towers are in that area?
And there are also drawbacks, he added. If the suspect lives or works near the crime scene, the tower data isn't effective. And if one tower isn't working, it could transfer numbers to another tower, which could place someone closer to a location than they actually were.
Michael Bailey, a retired Pinellas sheriff's homicide detective, said the technology is a tool that leads police to other information, like disproving alibis. For example, if a suspect says he was out of town at the time of the murder, but cellular records show he was actually near the scene.
"There are some variables in it, and it's never going to be a standalone," Bailey said.
Some cases involving tower data evidence have been challenged, including one involving a Portland woman convicted in 2004 of killing her lover. In an appeal, an expert said prosecutors didn't consider the wide range covered by the tower that picked up her phone on the day of the murder. A judge vacated her guilty plea in 2014, according to the National Registry of Exonerations.
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Collecting cellphone data as an investigative tool has also raised privacy issues. On Wednesday, the U.S. Supreme Court heard a case, Carpenter vs. United States, that will determine how the Fourth Amendment protects this kind of data.
In the Carpenter case, Michigan investigators got phone records for suspects in a robbery investigation without a warrant.
One suspect, Timothy Carpenter, was convicted based in part on the cellphone evidence, according to the American Civil Liberties Union. His appeal was denied, prompting the ACLU to petition the U.S. Supreme Court.
Contact Laura C. Morel at email@example.com. Follow @lauracmorel.