Gunshots broke out shortly after 6 p.m. Thursday, and within moments Baltimore police officers nearby received an alert on their cellphones.
The message told them gunfire was detected at 6:08 p.m. on Eutaw Place in Bolton Hill. A map of the location appeared on their phones. Officers arrived within seconds.
Those shots were the first recorded by Baltimore’s new gunshot detection system, a series of audio sensors on streetlights and rooftops listening to five square miles of West Baltimore. On Friday, police announced that the long-awaited ShotSpotter network had recorded four incidents of gunfire during its first night on the job.
“It’s going to aid us significantly in the crime fight,” interim Baltimore Police Commissioner Gary Tuggle said.
Police and city leaders have tried for years to bring a gunshot detection system to Baltimore. ShotSpotter went live Thursday at 5 p.m. Police found no victims from the first four incidents. But they found shell casings from gunfire detected 1:37 a.m. Friday on North Bentalou Street.
The network relies on dozens of audio sensors installed 30 to 40 feet off the ground. Each sensor records the sound, time and location of sudden noises like booms and bangs. These recordings are filtered through computer algorithms and screened by listeners day and night at ShotSpotter headquarters in California and by police analysts in Baltimore. They send the cellphone alerts within seconds. Sensors retain the recordings for 72 hours.
Across the country, police are increasingly turning to gunshot detection systems. Cincinnati police began using ShotSpotter in August. They said most gunshots go unreported and neighbors called 911 in only about one in six incidents.
Baltimore officials have considered installing an audio shot-detection system several times during the last decade, and backed out of previous initiatives to purchase the equipment. The city abandoned plans in 2015 to buy gunshot detection equipment, despite a $305,000 state grant to help cover the costs. Officials said at the time that the cost to use the technology would take away from other crime-fighting efforts.
But as the technology has become more reliable, police say it is a powerful new tool in crime fighting. ShotSpotter will be a cornerstone of the new police nerve centers opening in East and West Baltimore. The centers will also use real-time data to pinpoint crime hot spots and dispatch officers there — a model known as predictive policing. Baltimore police plan to roll out ShotSpotter across five square miles of East Baltimore by the end of this month. They say the technology will alert offices to a shooting in about 45 seconds.
“We’re trying to make our response time a lot quicker,” Tuggle said.
He stressed that the system is not intended to replace 911. Tuggle urged residents to continue calling police if they hear gunfire.
Reviews of the technology have found that frequent alerts do not necessarily translate to arrests. The technology does not always lead police to the right location, and other loud noises, such as fireworks, can trigger alerts.
Some problems were uncovered by a critical report by the Center for Investigative Reporting. The report found that of the 3,000 alerts issued by ShotSpotter in San Francisco over 2 1/2 years, two arrests were made; one was gun-related. Five cities with ShotSpotter have let their contracts run out, including Charlotte, N.C., and Quincy, Wash., according to the report.
Baltimore police, however, say they tested the technology extensively and placed the sensors in strategic locations to avoid the false positives from loud noises echoing among downtown buildings. They also said the recordings have proven admissible in court.
Police received an $860,000 grant from Bloomberg Philanthropies to pay for ShotSpotter in East and West Baltimore for one year. They will review the results after the year and decide whether to continue and expand.
Mayor Catherine Pugh said in a statement that she’s hopeful ShotSpotter will help reduce gun violence in the city, which has seen 111 homicides this year to date. Of those killings, 99 have been shooting deaths.
“We are confident that the implementation of this sophisticated intelligence gathering capability will enhance our efforts to get illegal guns and criminals off our streets,” Pugh said.
ShotSpotter is used in more than 90 cities in the U.S. Baltimore police provided statistics that show Camden, N.J., and Springfield, Mass., saw gunfire reduce nearly 50 percent since deploying ShotSpotter.
The Associated Press contributed to this article.