The Denver Police Department purposefully fired gunshots in four Denver neighborhoods Monday night.

They were to test the city’s ShotSpotter system by firing dozens of rounds into a special target that absorbed the bullets.

The shooting lasted from 7 p.m. to 11 p.m. in the Westwood, Villa Park, Clayton and Montbello neighbothoods using two separate pistols and a rifle.

In a press release, DPD said, “During the tests a bullet trap will be used to ensure public safety—no bullets will be fired into the air or ground, and there is no danger to the public.”

DPD handed the press release out to homes in the four neighborhoods so residents were aware of the gunshots, and not to be alarmed.

The ShotSpotter system was implemented in Denver in 2015. Since, DPD says it has led to 98 arrests and 78 gun confiscations across the city.

DPD does not divulge its ShotSpotter locations as a way to maintain the integrity of the program.