SAN FRANCISCO — Minutes after a shooting near the Oakland Airport this year, the gunman was on the loose. And police Sgt. Chris Bolton quickly fired off a flurry of text alerts to thousands of nearby residents through a social media tool for law enforcement agencies.

“Stay out of area,” said one alert. “Multiple shooting victims reported. Medical on-scene. Police are evacuating a nearby, affected business.”

Officers would eventually discover a grisly scene inside a tiny Christian college on that spring day. Seven people were killed, three others wounded and dozens terrified in the deadliest mass shooting in the city’s history.

Bolton later gave those on edge an update: “Possible suspect in custody. No imminent public safety threat appears to exist in immediate area.”

Across the country, law officers are adding a new form of social media to their arsenal of crime fighting tools.  

Almost 6,000 law enforcement agencies are now deploying the public notification service Nixle to provide residents with real-time alerts on crimes in progress, traffic messes and missing children. Previously, the service has helped police in Amarillo, Texas, capture a fugitive wanted for aggravated robbery and probation violation; and authorities in Fayetteville, N.C., nabbed a suspect wanted for armed robbery soon after a Nixle alert was sent to residents.

With the San Francisco-based service approaching 1 million subscribers, with police departments participating in major cities such as Chicago, Los Angeles, Baltimore and Dallas, it is part of what one expert calls a new “blue wave” of electronic community policing that lets cops reach out directly to the public.

The International Association of Chiefs of Police recently found in a survey of 800 law enforcement agencies that nearly nine out of 10 use some form of social media and more than half reported that social media have helped solve crimes.

Many of those agencies have boosted their social media presence significantly within the last two to three years, said Nancy Kolb, a senior program manager for the IACP.

“I think there was some initial hesitancy due to safety and security reasons and because some new fads in technology don’t always take hold,” said Kolb, adding that the IACP established its Center for Social Media in 2009 after many police chiefs sought guidance on how to incorporate the technology into their departments.

“Slowly, some saw what their counterparts were doing and the really successful ones are able to find out what works specifically within their departments that may not work well in others.”

While many law enforcement agencies use Twitter and Facebook, and there are other public safety notification systems such as CodeRED, Nixle has become increasingly popular with law enforcement, said Lauri Stevens, a Massachusetts-based law enforcement social media strategist.

Police officials say they particularly like Nixle security features that make it less susceptible to hacking than some social media. Also, the company has a partnership with the National Law Enforcement Telecommunications System (NLETS), which allows local, state, federal and international public safety agencies to exchange sensitive information.

A private messaging system Nixle created has allowed authorities to use it at major events including the G-20 Summit, May Day immigration protests and even the Academy Awards.

However, Nixle is not as interactive as some of the other social media networks, Stevens said.

“The company has taken off like crazy and it helps law enforcement send information out, but it can’t get any answers back, so I don’t consider them social media in the true sense,” she said. “What Nixle has done right, though, is that they’ve got good buzz and are still building a huge customer base.”

With about $10 million in startup funding, Nixle began in 2007 as a potential competitor to the hyper-local news site Patch, but instead found its niche a year later by providing those in law enforcement a network to communicate directly to the public.

The company says it hasn’t yet turned a profit, CEO Eric Liu said, but he hopes that police departments that currently get its service for free will later sign up for paid premium services and upgrades, such as a secured anonymous tip service through the Web.

“Typically the community’s experience with public safety is usually negative, but Nixle is about creating positive experiences and I think that’s important to build,” Liu said. He said the police agencies have been “hands-on, and have literally helped build the product.”

The system allows police departments to send out alerts and advisories to its subscribers either citywide or just to a specific neighborhood within a quarter-mile radius. A message can be received either through text or e-mail, and can also be sent out through other social media platforms.

Subscriptions typically spike when populaces suffer crises such as tornadoes, wildfires and major violent crimes, Liu said.

“But we still need to get smarter about how we interface with social media,” Liu said. “We are reaching out to more people than we realize.”

On April 2, when a former student opened fire on students at Oikos University in Oakland, Bolton found Nixle extremely important in providing residents real-time information to a city on edge.

“I’m no different than any other police officer wanting to do everything you can to intervene and help during a crisis,” Bolton said. “There’s no way I can compare the stress and desire to do that from a computer terminal to that of the officers who bravely entered that campus to save lives, but I think social media alert tools serve a unique public safety purpose just the same.”