MOUNTAIN VIEW, Calif. -- At 7:22:07 p.m. on a recent Thursday evening, an electronic alarm went off in the soundproofed control room of a suburban office building here.
A technician quickly focused on the computer screen, where the words "multiple gunshots" appeared in large type. She listened to a recording of the shots -- the tat-tat-tat-tat-tat of five rounds from a small caliber weapon -- and zoomed in on a satellite map to see where the gun had been fired: North 23rd Street in Milwaukee, 2,200 miles away.
At 7:23:48, the technician, satisfied that the sounds were gunshots, sent an alert to the Milwaukee Police Department. Less than two minutes later -- or 10:25:02 p.m. Wisconsin time -- a tactical team arrived at the address to find five .22-caliber shell casings and a bleeding 15-year-old boy who had been shot in the arm. The casings, said Chris Blaszak, a detective assigned to the department's intelligence fusion center, were found within 17 feet of where the alert had placed the shooter. Total elapsed time: 3 minutes, 55 seconds.
Milwaukee is one of an increasing number of cities around the country -- just under 70 to date, including some in the New York area -- that are using a gunshot detection system, called ShotSpotter, to pinpoint the location of gunfire seconds after it occurs. Last year, the company that developed ShotSpotter began offering a more affordable system, and that has brought in new clients and led other cities to consider trying it.
The detection system, which triangulates sound picked up by acoustic sensors placed on buildings, utility poles and other structures, is part of a wave of technological advances -- among them, license plate scanners, body cameras, Global Positioning System trackers and hand-held fingerprint identifiers -- that is transforming the way police officers do their jobs. But like other technologies, the gunshot detection system has also inspired debate.
In at least one city, New Bedford, Mass., where sensors recorded a loud street argument that accompanied a fatal shooting last December, the system has raised questions about privacy and the reach of police surveillance, even in the service of reducing gun violence.
And with recession-plagued police departments having to cut personnel and services, some cities have questioned the system's benefits relative to its cost. Detroit's City Council last year rejected the police department's proposal for a three-year, $2.6 million contract, with one City Council member objecting that not enough officers were available to respond to the alerts.
Cities that installed ShotSpotter in the past bought the equipment and managed the alerts themselves -- a model that often involved outlaying hundreds of thousands of dollars. But the company now offers a subscription plan for a yearly fee of $40,000 to $60,000 per square mile that includes round-the-clock monitoring of alerts by trained reviewers here in Mountain View.
Many police officials say that the system has significantly improved response time for crimes involving firearms and, by demonstrating that the police can show up quickly at the right place, has increased community confidence and helped deter gun crime. The technology, they say, has provided officers with critical information about what to expect upon arriving at a crime scene -- among other things, whether if a gun was fired from a car and if so, how fast and in what direction the car was traveling -- and offered a level of precision in locating gunfire rarely afforded by 911 calls.
Sgt. Chris Bolton of the Oakland Police Department, which has installed ShotSpotter in high-crime neighborhoods in East and West Oakland, said that before the system was in place, "a patrol officer would receive a gunshot call from the community and you could spend up to 30 minutes driving within, I would say, three to four blocks of that location, just to make sure there isn't a victim in need of assistance, a crime ongoing or any evidence."
If nothing else, ShotSpotter has made it clear how much unreported gunfire takes place on city streets. In many high-crime urban neighborhoods, gunshots are a counterpoint to daily life, "as common as the birds chirping," as Commander Mikail Ali of the San Francisco Police Department put it. But whether out of apathy, fear or uncertainty, people call the police in only a fraction of cases.
In San Francisco's Bayview-Hunter's Point neighborhood, for example, where one square mile is covered by ShotSpotter sensors, only 10 percent of the verified incidents of gunfire detected by the system were accompanied by 911 calls, Commander Ali said. In Oakland, according to Sergeant Bolton, only 22 percent of the verified gunfire the system detected over a three-month period was also reported by citizens.
Chief Chris Magnus of Richmond, Calif., a community of 120,000 north of Berkeley that routinely ranks among country's most violent cities, recalled listening to a ShotSpotter recording of a gun battle in 2010 that involved more than 100 rounds fired from four guns.
"It was just mind boggling," he said. "This is like 11 at night on a summer night, and nobody even called it in."
As the technology has evolved -- it was first developed in the 1990s by an engineer, Robert Showen, who hoped it might help address a spike in gun-related homicides in East Palo Alto, Calif. -- it has become more accurate, the company says, with fewer false positives and false negatives. The challenge for the system is to distinguish rounds of gunfire from other sharp noises -- backfires, construction, firecrackers, the thwap-thwap-thwap of a helicopter's propeller. ShotSpotter's alerts label the recorded event -- identifying it as a single gunshot, for example, or a backfire -- and attach a probability that the identification is correct.
A 2006 study financed by the National Institute of Justice of test shots fired at the Charleston Navy Yard, conducted at the company's request, found that ShotSpotter correctly detected 99.6 percent of 234 gunshots at 23 firing locations. The system also located 90.9 percent of the shots to within 40 feet.
Still, some criminal justice experts say that how well the technology works and how essential it is to police departments has yet to be proved.
"Whether this will be seen long-term as a short-term law enforcement fad or fundamental to the way police work, that, I think is the question," said Peter Scharf, a criminologist at Tulane University. "I don't think the effectiveness or efficiency arguments have been settled quite yet."
But Chuck Wexler, executive director of the Police Executive Research Forum, a nonprofit group in Washington, said that especially in cities like Richmond, where gun violence is frequent and police response time can make a difference, the use of ShotSpotter makes sense. "I think it's a real advantage," he said.
Over the course of several hours on two recent evenings in the control room here in Mountain View, reviewers listened to recordings identified as gunfire, backfire or firecrackers in 13 cities, including Oakland, Calif., Panama City, Fla., Wilmington, N.C., and Milwaukee, deciding in each case whether an alert was accurate. Those judged to be valid were sent on to the cities' police departments.
Sgt. Eric Smith of the Richmond Police Department said that in ShotSpotter alerts, he has heard in the background "doors slamming, birds chirping, cars on the highway, horns honking."
In New Bedford, the ShotSpotter recording of a street argument is likely play a role in the case against two men, Jonathan Flores and Jason Denison, charged with murder in the Dec. 2, 2011, killing of Michael Pina.
At a bail hearing in January, an assistant district attorney said that the system had recorded arguing and yelling on the corner of Dartmouth and Matthew Streets.
Frank Camera, the lawyer for Mr. Flores, said that if the prosecution used the recording as evidence, the issue of privacy could be raised under the state's wiretapping statute. Mr. Denison's lawyer, Kathleen Curley, said she planned to file a motion to that effect on behalf of her client.
Mr. Camera said that whether he, too, would argue that the recording constituted a privacy violation depended on what is on the tape.
In one section, he said, a voice can be heard saying "No, Jason! No, Jason" -- a statement that could help his client -- but in other portions the words cannot be easily distinguished. He said he was having the tape enhanced to try to clarify it.
In any case, Mr. Camera said, the new technology is "opening up a whole can of worms."
"If the police are utilizing these conversations, then the issue is, where does it stop?" he said.
Sam Sutter, the district attorney in Bristol County, Mass., called ShotSpotter "an extremely valuable tool" that had helped his office bring charges in four nonfatal shootings.
"In my view legally," he said, "what is said and picked up by the Shotspotter recording does not have the expectation of privacy because it's said out in public, and so I think that will turn out to be admissible evidence."
James G. Beldock, a vice president at ShotSpotter, said that the system was not intended to record anything except gunshots and that cases like New Bedford's were extremely rare. "There are people who perceive that these sensors are triggered by conversations, but that is just patently not true," he said. "They don't turn on unless they hear a gunshot."
This article originally appeared in The New York Times.