A double murder that rattled a quiet street in Metairie. A sexual assault in which the primary suspect insisted he had been nowhere near the crime scene. A convicted felon driving an ominous van seemingly outfitted for an assassin.

These disparate crimes, all of which occurred in Jefferson Parish, share a common denominator: In each case, the use of automatic license plate readers, a controversial technology that’s becoming increasingly common around the state, helped law enforcement bag their suspects.

The Jefferson Parish Sheriff’s Office operates a growing network of them — more than 100 stationary cameras, mounted along various thoroughfares in the parish — that Sheriff Newell Normand describes as the largest in the state.

Several other law enforcement agencies, including the State Police, rely on the devices not only to track down stolen vehicles but also to trace the movements of fugitives and persons of interest, compiling massive databases of images that eventually are purged or overwritten unless they’re found to hold some value as evidence.

As the practice becomes more ubiquitous, it is giving rise to a familiar modern quandary of how to balance the obvious benefits of a technology that has helped break high-profile criminal cases with concerns about how closely law enforcement can track individuals who have not even been accused of a crime, let alone convicted of one.

The State Police maintain 35 stationary license plate readers around the state that have been credited for significant drug interdictions and the recovery of missing children. Authorities say those cameras, which generate some 9 million images a month, can prove invaluable when they are searching for a specific vehicle or know just part of a license plate number.

“Let’s say my child was missing. I want you to do whatever you can to get my child back,” said Col. Mike Edmonson, the State Police superintendent. “I want to use every tool available to me to find that perpetrator or to affect an arrest. God forbid it be a small child that’s abducted by a sexual predator.”

License plate readers could become even more prevalent under legislation passed by the Louisiana Senate last month that would target uninsured motorists.

But for all of its investigative benefits, the technology has been widely criticized by advocacy groups such as the American Civil Liberties Union. The critics have faulted law enforcement for releasing too little information about license plate reader programs and for retaining unneeded data for too long.

The Jefferson Parish Sheriff’s Office stores license plate images for three months before discarding the information, while the State Police program purges its data every 60 days. The proposed state pilot program similarly calls for a 60-day retention period.

Marjorie Esman, executive director of the ACLU of Louisiana, acknowledged the investigative advantages of license plate readers. But she said there’s no reason for law enforcement to retain the data more than a few days, warning of a “surveillance state.”

“The idea of a government database that has a record of who was where and when, subject to (the authorities) sending an inquiry at any time, means that the government can follow you around for 90 days,” Esman said, referring to the retention schedule in Jefferson Parish. “That’s terrifying. That’s Big Brother, and for the government to follow you around without a warrant is, arguably, unconstitutional.”

Dave Maass, an investigative researcher with a San Francisco-based electronic privacy group, the Electronic Frontier Foundation, noted that automatic license plate readers can reveal a wealth of information about a person’s daily routine and interests based on where they drive.

“It only takes a few data points to know where you work and where you live,” Maass said. “It can reveal a lot about your political affiliations. If police want to know where a reporter’s going, all they need is their license plate.”

A host of uses

The high-speed cameras, which Jefferson Parish first deployed in late 2008, capture images of passing license plates, often relying on optical character recognition and infrared lighting. The plate numbers and letters are automatically converted to computer-readable data and checked against national and local databases of wanted criminals and stolen vehicles.

Normand, the sheriff, receives an alert on his smartphone when a camera registers a “hit.” False readings occur regularly, requiring the authorities to manually check the images against a wanted license plate number.

“Within seconds, it tells us whether the vehicle has been stolen and sends us an alert,” Normand said. “But it’s much more than stolen vehicles. We’re putting people in places vis-a-vis the historical data generated by the license plate recognition cameras. We actually use them for internal investigations as well.”

Deputies respond quickly to such alerts, as they did on a Wednesday evening last month when a stolen vehicle, allegedly driven by Brandon Martinez, 18, was photographed entering Lafreniere Park.

A deputy tried to pull Martinez over, authorities said, but he began running stop signs and swerving around other vehicles. The chase ended after Martinez crashed the stolen car and ran into the bathroom of a nearby Waffle House, where he was taken into custody.

While several other investigative techniques were used, license plate readers also helped detectives make an arrest this year in the double murder of David and Nicholas Pence in Metairie, an investigation that highlighted the ability authorities have to search license plate reader networks for a specific type of vehicle.

Investigators knew there had been burglaries in the neighborhood and that a suspicious white SUV had been seen driving in the area. They were able to read the SUV’s license plate number from private home surveillance footage and plug it into the software that tracks license plates on their own network of cameras.

Once the vehicle’s license plate had been flagged, investigators could track its progress as it passed other readers in the network. That, combined with a fingerprint found at the crime scene, helped lead to an arrest.

License plate readers also played a role in the case of Jaren Lockhart, the Bourbon Street dancer whose dismembered remains washed up on the beach in Hancock County, Mississippi, in 2012. The devices allowed authorities to flag the vehicle of the suspected killers on Interstate 10 in Kenner and again as it left the state hours after the murder. The timeline, authorities have said, “offers a very clear picture of Jaren’s last hours and the immediate efforts of her murderers to cover their tracks.”

A growing presence

Esman, the ACLU executive director, said there does not appear to be a comprehensive list of agencies using license plate readers. Interviews and news accounts show, however, that they are being used in a growing number of Louisiana communities, from Bossier Parish to Baton Rouge. LSU police have two mounted license plate readers on campus that are connected to the National Crime Information Center.

The use of license plate readers would increase under legislation proposed this session by state Sen. Ronnie Johns, R-Lake Charles. Senate Bill 250, which sailed through the Senate last month and is scheduled for debate Monday in the House, would establish a statewide pilot program that would use license plate readers to catch uninsured motorists, linking the devices to the state’s vehicle registration and compulsory motor vehicle insurance databases.

Johns, an insurance agent, did not return messages seeking comment on the bill.

The growth of plate reader networks is somewhat limited by their cost. While some places like New Orleans have received grants to purchase readers, they still must spend money on maintaining image databases. The Jefferson Parish Sheriff’s Office spends roughly $200,000 a year to maintain its system, and the State Police spend about $90,000.

While most parishes are not yet saturated with mounted license plate readers, a growing number of jurisdictions are deploying a mobile version of the technology: cameras attached to police units that are constantly scanning the plates of both moving and parked vehicles.

The New Orleans Police Department, for instance, has seven mobile license plate readers but no stationary ones.

Six of the NOPD’s geographic districts, as well as the privately funded Mid-City patrol, have vehicles equipped with mobile scanners. The department has read plates 1.1 million times so far this year, leading to the recovery of 42 vehicles and four arrests, said Sgt. Danny Kramer, the commander of the NOPD’s Property Crimes Investigative Services Bureau.

Auto theft is the main target for the license plate readers, but the department also uses them to search for vehicles linked to Amber Alerts — child abduction cases — and to catalog cars seen near murder scenes. Every time they scan a license plate, those readers place the information about what car was seen and where into a database, where the information is held for 180 days — significantly longer than in Jefferson Parish or at the State Police.

That retention period is necessary, Kramer said, despite privacy concerns. “It could be a homicide case where you get a tip four months later — ‘It was so-and-so, and we think he drives this kind of car,’ ” Kramer said.

Privacy safeguards

License plate readers for years have raised concerns about the potential for abuse among law enforcement. There is no requirement in Louisiana that authorities first obtain a warrant to query their license plate reader networks or that the target even be under criminal suspicion. On this score, law enforcement officials say there is no expectation of privacy when you are driving on a public roadway.

At a recent NOPD commanders’ meeting, days after a missing mother was found dead with her children in a murder-suicide in New Orleans East, Sgt. Stacey Pearson, the coordinator of Amber Alerts for the State Police, hailed the network’s potential to prevent future tragedies.

“I can check a license plate reader for you from the comfort of my home,” Pearson said at the meeting. “I can get a license plate entered into the license plate system very easily. I have contacts throughout the nation, especially in our neighboring states.”

Both of the state’s major east-west interstates, I-10 and I-20, have plate readers focused on them. State Police can also query Texas and Mississippi databases upon request, according to Pearson.

It is those kinds of wide-ranging capabilities that raise concerns for people like Maass at EFF, the privacy group.

“If you had a corrupt mayor who could get access to this database, he could get information on his opponents,” Maass said.

The State Police, the JPSO and the NOPD all say they have taken measures to prevent abuse of license plate reader databases, including limiting access to them.

In New Orleans, Kramer said, only he and one other officer can query the license plate database. Ordinary troopers cannot query the State Police database, Edmonson said. In Jefferson Parish, searches are run out a central criminal intelligence office.

“There’s only certain folks that can go and query the (license plate reader) system,” Normand said. “It’s not open to everybody.”

The idea of the network, he said, is not for deputies to be able to snoop on ex-girlfriends and in-laws. Every search run on the database is recorded in an audit trail that can be reviewed later, a safeguard against the potential for misuse.

“You’re only as good as the weakest link in the chain when you want to keep anonymity and control,” Normand said.

Follow Jim Mustian on Twitter, @JimMustian. Follow Matt Sledge on Twitter, @mgsledge.