Law enforcement officials and civil libertarians debated a bill Thursday that would limit how police use a tracking device that can locate a cellphone — and its user — to within six feet.

Bipartisan legislation in the House of Delegates would require police to obtain a search warrant to use the cell site simulator, known as a “stingray,” to track a suspect. It also would impose strict requirements that officers filter out and discard information about other cellphone owners caught up in any sweep.

Proponents of the bill say it is needed to protect individuals' Fourth Amendment right against unreasonable search and seizure. Police and prosecutors say the bill is unnecessary and would hobble a vital tool for fighting crime.

The Baltimore Sun reported last year that the city's Police Department had used the controversial technology thousands of times in recent years and hid that fact from prosecutors and judges, at the FBI's request. Del. Charles E. Sydnor III, lead sponsor of the bill, said it was prompted in part by articles in The Baltimore Sun.

Sara Love, a lawyer with the Maryland ACLU, told the House Judiciary Committee that privacy concerns outweigh the technology's usefulness to police.

“Breaking into everybody's home and searching it also would help them catch bad guys, but we have a Constitution,” she said.

But Scott Shellenberger, the state's attorney for Baltimore County, told lawmakers the bill is based on “misinformation” about a technology that locates only a cellphone's unique identification signal.

“It does not listen to your phone call. It does not capture your text. It does not capture email. It doesn't capture anything that's in your phone,” he said. Shellenberger was joined in opposition by witnesses from the state police and the Baltimore and Baltimore County police departments.

Typically carried in the back of a police van, a stingray acts as a fake cell tower, gathering the identifier numbers — not the same as the phone numbers — of each mobile device with a given carrier.

Shellenberger said the stingray goes into an area looking for the unique identifier and not other phones.

“If it doesn't get a hit, it redirects that signal to its regular cell tower,” he said. Shellenberger said the other cellphone information is purged right away.

The bill would require police to take steps to ensure that no data is collected that is not authorized by a judge based on probable cause.

“Everybody realizes that while police have a great responsibility in protecting us all, we don't want unwarranted intrusions — that there is a Fourth Amendment,” said Sydnor, a Baltimore County Democrat.

In recent years, judges have been skeptical of stingray use. Last year, a Baltimore police detective revealed in court that he had used the device thousands of times while withholding information about it from prosecutors and judges under instructions from the FBI.

When a defense lawyer pressed for more information in the case of a defendant charged with stealing a cellphone, the detective balked, citing the FBI confidentiality agreement.

Baltimore Circuit Judge Barry G. Williams ordered the detective to answer questions, saying, “You don't have a nondisclosure agreement with the court.” Prosecutors then dropped the use of the evidence provided by the stingray.

The bill would forbid law enforcement agencies from using the device if “an inordinate number” of other people's personal information might be collected. It would prohibit wide searches in residential areas when police do not know where the target cellphone is — something police said would shut down such operations. The bill also would require that police persuade a judge to issue a warrant to use the device. Shellenberger said police now get a court order before using a stingray that is much like a warrant and uses the same probable-cause standard.

The bill has attracted bipartisan support, with Baltimore County Republicans Robin L. Grammer Jr., Ric Metzgar and Christian Miele signing on a co-sponsors.

Grammer said such legislation is needed in “the age of the surveillance state.”

mdresser@baltsun.com