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: just less than four minutes.

Milwaukee is one of an increasing number of cities around the country — just under 70 to date, including Quincy, Wash. — 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 handheld fingerprint identifiers — that is transforming the way police officers do their jobs. But like other technologies, the gunshot-detection system also has 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 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.

Officials in Quincy, Grant County, paid $130,000 for the software program after three gang-related fatalities in 2011.

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 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, Calif., 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 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, said Commander Mikail Ali of the San Francisco Police Department. In Oakland, according to Bolton, only 22 percent of the verified gunfire the system detected over a three-month period was 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 gunbattle 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 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; 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.

Information from The Associated Press is included.