NYPD Commissioner William Bratton was expected to announce Monday an expansion of the ShotSpotter gunfire-detection system in sections of the Bronx and Brooklyn, a department official said.

The announcement of additional acoustic detectors that use triangulation to locate potential shooting locations comes at a time this year when shootings have climbed, particularly in those two boroughs.

While the latest shooting statistics won't be available until later Monday, NYPD data through March 8 showed shootings up more than 21 percent so far this year from the same point a year earlier, driven primarily by what Bratton has described as gang activity and cases in which marijuana trafficking has been a factor.

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The NYPD has been testing ShotSpotter technology since 2011 and last year deployed a few acoustical sensors in the Brooklyn area where, on Dec. 20, they picked up the sounds of gunfire associated with the killings of Officers Wenjian Liu and Rafael Ramos, investigators said. Liu and Ramos were shot dead as they sat in their patrol car in the Bedford-Stuyvesant neighborhood by Ismaaiyl Brinsley, who later killed himself.

The system was to go operational at midnight Sunday in seven Bronx precincts and seven in Brooklyn.

ShotSpotter technology, a product of California-based SST Inc., has been used in several Long Island communities, including Huntington Station, North Amityville and Hempstead. Bratton had been a member of the company's board, but left by the time he became police commissioner in January 2014.

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A study released in 2013 of the experience with ShotSpotter in Suffolk gave mixed reviews. Of 212 potential shootings picked up, about one third were found to have been false alarms and the bulk of the remainder could not be substantiated, the study found. Only 7 percent of the alarms turned out to be verified shootings, the report found.

SST officials have said their system provides data, and it is then up to the police agencies to respond and gather evidence.

NYPD officials say the technology can better focus responding officers so they can get to a shooting scene faster, gather evidence and possibly apprehend suspects. Bratton signaled that the NYPD was considering wider ShotSpotter use during a City Council hearing last year.The technology also has been in use in several U.S. cities, including Oakland, California; Miami; Omaha and Washington, D.C., according to the company website.