One morning in October 2010, a young man showed up wounded at a hospital in Richmond, saying he'd been robbed and shot nearby.

What 21-year-old Brandon Wallace didn't realize was that Richmond police had blanketed the city with a gunfire alert system called ShotSpotter - and the brigade of sensors had registered no blasts that morning.

However, there had been a slaying that morning in nearby Berkeley in which one suspect had accidentally shot another in the rear end. When police issued an all-points bulletin for a man wounded in such a way, Wallace was in trouble. He was soon booked on suspicion of murder.

The case showed how ShotSpotter has become a basic component of police work in six Bay Area cities that have bought it in recent years. While offering mostly anecdotal evidence, and acknowledging some problems, commanders in the cities say it's been a success and is likely to be a permanent feature of their jobs.

The cities include San Francisco, which is in the midst of expanding its network from 4.3 to 9.3 square miles, and Oakland, which restarted its system recently after failing to pay maintenance fees and allowing the sensors to fall into disrepair. East Palo Alto, Redwood City and San Pablo also use ShotSpotter.

Its proponents say the system enables officers to respond faster, often reaching the spot where shots were fired before the first 911 caller dials in. In many rough neighborhoods, officials said, the vast majority of gunfire prompts a ShotSpotter alert but no human calls.

Basic information that officers formerly labored to gather - such as how many shots were fired - arrives in seconds. Police are also using ShotSpotter as a planning tool, identifying hot spots where extra patrols are needed.

As he displayed a series of ShotSpotter alerts on his office computer recently, replaying the recorded bangs and booms, San Francisco police Cmdr. Mikail Ali said the system also did something less tangible: convince people that police were on their side.

"We used to miss a lot of these (gunfire) incidents," Ali said. "People assumed we knew about them, but we didn't. They would say, 'They don't show up because they don't care.' It further polarized communities."

Rooftop ears

ShotSpotter, which is made by SST Inc. of Mountain View and is used in 60 cities nationwide, is a relatively simple technology. Sensors that can distinguish between "acoustic events" are perched on rooftops and utility poles, with 15 to 20 per square mile.

Sound waves from a gunshot reach each sensor at a different time, allowing for triangulation. Police officials said the system is precise, with a margin of error of about 5 yards, but isn't designed for indoor shootings.

With multiple gunshots,the system can tell if they were fired from a moving car and estimate the car's speed and direction.

Ralph Clark, SST's chief executive, said a lot of gunfire represents drug dealers marking turf by instilling fear.

"Gunfire is good for business," Clark said. "We want to change the nature of that ... so that when there's gunfire, the police show up quickly and precisely to where the gun was fired."

When cities began installing the sensors, though, critics feared they wouldn't justify their price tag.

Buying the equipment costs $200,000 to $300,000 per square mile, plus an annual maintenance fee of roughly 15 percent of that total. A subscription-based option the company began offering more recently is $40,000 to $60,000 annually per square mile.

Police watchdogs - and many officers, as well - envisioned shifts clogged by wild goose chases, with the sensors mistaking sounds like fireworks and construction noise for gunfire.

Such false alarms remain a critical issue for SST. In a report the company commissioned earlier this year, dispatchers in cities with ShotSpotter said they believed about one-third of gunfire alerts were bogus.

Management key

The problem underscores a broader reality: As with surveillance cameras, GPS ankle monitors and other high-tech tools that police have embraced in recent years, ShotSpotter is only as good as the human beings who act on its data.

Clark said dispatchers at some agencies used to send officers to check on every noise captured by the system. In some cases, he said, they didn't pass along key details from a ShotSpotter alert, such as if there were multiple shots - potentially sending the officer unprepared into a gunfight.

It was a big enough problem that the company launched around-the-clock "managed service." For cities that subscribe - San Francisco and Oakland now do, though Richmond does not - workers at a ShotSpotter command center analyze alerts as they come in to make sure they are legitimate, then send them to police in about 20 seconds, along with brief notes.

The police leaders said the false alerts have been outweighed by benefits, and that they and their dispatchers had grown more skilled in recognizing legitimate gunfire. On ShotSpotter's software, it produces a unique sound wave, appearing like a sideways Christmas tree.

Hits and misses

A Richmond police captain, Mark Gagan, said his city's sensors detected 218 incidents of gunfire in November. The system and city employees filtered out more than 600 other sounds, such as train horns, firecrackers and car backfires.

In many cases, Gagan said, officers quickly found evidence like shell casings that could be compared to other shootings.

In one case two years ago, he said, officers tipped by ShotSpotter found two bleeding victims. No one had called 911, and one of the men might not have survived without the alert.

As Gagan spoke, an alert came in - five gunshots. A lieutenant, Bisa French, replayed the noise on her computer, looked at the map of the location and the sound wave and said, "Those aren't gunshots." In seconds, her screen showed that a dispatcher had reclassified the alert as "construction noise."

Officer Matthew Stonebraker, a five-year veteran who grew up in Richmond, said later that he was often sent on false calls but didn't mind them.

"It's helpful to all of us out on patrol," he said. "The guesswork is taken out of responding to a call. They give you a location, a map, an address of where it actually occurred."

A big question in many cities now is whether ShotSpotter should be combined with other surveillance, such as cameras and license plate scanners.

In Richmond, retired police Officer John Hugel, who was hired to keep an eye on the city's network of surveillance cameras, showed how he checks after an alert from ShotSpotter to see if any of the cameras are nearby. If one is, he can maneuver it remotely in an effort to spot a shooter or a getaway car.

Studies needed

Assessing whether ShotSpotter has paid off, though, remains difficult. It hasn't been independently studied, and police officials in the Bay Area cannot say exactly how often alerts are valid, nor how often they lead to an arrest or a seizure of a weapon.

The ShotSpotter-commissioned study estimating that a third of gunfire alerts were false alarms relied on interviews with system users, not hard data.

It makes sense that police would want to know about gunfire more quickly and more often. After all, nearly 70 percent of California's homicide victims in 2010 were shot.

"The question is, what good does it do and in what kind of cases?" said UC Berkeley criminologist Franklin Zimring. "What happens after you locate the shots?"

ShotSpotter sensors are dormant most of the time. But in rare cases - after awakening because of gunfire - they have captured critical sounds nearby.

In June 2007, 37-year-old Tyrone Lyles was gunned down under a ShotSpotter sensor in East Oakland. The recording preserved his final words: "Why you do me like that, Ar? Why you do me like that, dude?"

The head of homicide at the time, Capt. Ersie Joyner, believed that was a reference to a suspect in a previous killing, Arliton Johnson - whom a witness soon picked from a lineup. Johnson was convicted of murder.

Oakland's network later dropped out of service. According to a city report, police officials had failed to pay maintenance fees for two years - racking up a bill of $108,000 - when the City Council voted in October to cut a deal with ShotSpotter, spending $84,000 to restore it and expand it.

The new deal includes managed service from ShotSpotter's command center. Joyner, who is now point person on the project, said Oakland had been one of the cities that struggled to differentiate among the system's alerts.

"We just used it as a high-priced early-warning system," Joyner said. "Now there will be some thought that goes into it and a strategic response. Given our reduction in staffing, we need to have force multipliers."

Terrible sounds

Regardless of whether ShotSpotter prevents a shooting or prompts an arrest, it provides a window into the pervasive nature of gunplay in urban neighborhoods.

On Nov. 26, for example, the sensors picked up a barrage of 27 shots in central Richmond, then 13 more shots just down the street, then 20 more. The recordings are jarring.

"Shooter moving west at 16 mph," one of the alerts said.

Police, who would find three victims with bullet wounds, immediately suspected what was going on - gangsters from North Richmond cruising through rival turf, with a pair of shooters firing indiscriminately at people on the street.

Detectives are still hunting for suspects. ShotSpotter may be powerful, but it's not everything.

"Technology tells a vivid story," Gagan said. "But we still need people to come forward and help us."