A shooter at a bank in downtown Louisville, USA, has killed at least four people and wounded at least eight others on Monday, April 10 police said.

Police arrived as gunshots were still being fired inside the Old National Bank and exchanged fire with the shooter, Louisville Metro Police Department Deputy Chief Paul Humphrey said at a news conference. It wasn’t clear whether the shooter killed himself or was shot by officers.

4 killed in bank shooting at downtown Louisville building U.S.

The shooting, the 15th mass killing in the US this year, comes just two weeks after a former student killed three children and three adults at a Christian elementary school in Nashville, about 160 mile (260 kilometers) to the south.

“We believe this is a lone gunman involved in this, that did have a connection to the bank. We’re trying to establish what that connection was to the business, but it appears he was a previous employee,” Humphrey said.

4 killed in bank shooting at downtown Louisville building U.S.

Humphrey said that at least eight people were being treated at a hospital for wounds, including two police officers, one of whom was in critical condition.

An emotional Kentucky Governor Andy Beshear said he lost friends in the shooting in the building on East Main Street not far from the Louisville Slugger Field and Waterfront Park.

“This is awful,” he said. “I have a very close friend who didn’t make it today. And I have another close friend who didn’t, either. And one who’s at the hospital that I hope is going to make it through.”

It was the second time that Beshear was personally affected by a mass tragedy since becoming governor.

Humphrey, the deputy police chief, said the actions of responding police officers in Louisville on Monday morning had undoubtedly saved lives.

“This is a tragic event,” he said. “But it was it was the heroic response of officers that made sure that no more people were more seriously injured than what happened.”

This year’s 15 mass shootings is the most during the first 100 days of a calendar year since 2009, when 16 incidents had occurred by April 10, according to a mass killings database maintained by The Associated Press and USA Today in partnership with Northeastern University.