The blast of a shotgun split the foggy air next to Lake Merced on Wednesday afternoon, demanding the attention of onlookers, including Mayor Gavin Newsom.

Within five seconds, a computer system calculated the location of the shot to within feet inside the roughly 500-square-yard parking lot of the San Francisco Police Department’s training facility on the shore of the lake. Sensors picked up the shot, triangulated the location and marked it on an aerial photo of the lot on a computer screen.

The dramatic demonstration showcased the ShotSpotter system, a technology for which $400,000 was originally earmarked in this year’s budget but was placed on hold pending more detailed information from the Mayor’s Office. On Wednesday, the office asked for the funding to be taken off hold and spent on the system.

“Literally, lives can be saved. There’s an imperative not to wait,” Newsom said at a news conference to promote the system. He said 50 percent of gunshots fired in San Francisco streets go unreported. Meanwhile, he said, 85 percent of The City’s 85 homicides were committed with a gun in 2006, compared with 60 percent in 2001.

The City has recorded 81 homicides so far this year. At this time last year, 66 had been tallied.

The $400,000 in funding was put on hold after the Department of Technology and Information Services couldn’t answer what Supervisor Ross Mirkarimi characterized as “basic line-item-related questions regarding ShotSpotters, everything from performance to methodology to cost,” during a Board of Supervisors budget hearing.

“I believe the will is there to support this, but it needed the due diligence to be done,” said Mirkarimi, who has been a proponent of the system for nearly two years. “I think ShotSpotters will be a very basic strategy to help alleviate the gun violence,” he said.

The system costs about $159,000 per square mile, including hardware and installation costs, ShotSpotter Inc. CEO James Beldock said. The proposed deployment areas in The City would include the Western Addition and Bayview, two areas plagued by gun violence, Newsom said.

Beldock pointed to Redwood City as an example of a city that has successfully used the ShotSpotter system, saying that gun violence there has declined 70 percent since the implementation of the system 10 years ago.