A bank employee armed with a rifle opened fire at his workplace in Louisville, Kentucky on Monday morning, killing five people and wounding at least eight others while livestreaming the attack on Instagram.
The gunman, identified as Connor Sturgeon, was fatally shot at the scene, Louisville police said.
It was unclear whether he was killed by police or took his own life.
Two police officers were among the nine wounded. A 26-year-old recent police academy graduate was struck in the head and remained in critical condition after brain surgery on Monday, police said.
President Joe Biden responded to news of the shooting by reiterating his wish that Congress pass legislation requiring safe storage of firearms, background checks for all gun sales and elimination of gun manufacturers' immunity from liability.
Kentucky Governor Andy Beshear said he lost one of his closest friends in the shooting — Tommy Elliott — in the building not far from the minor league ballpark Louisville Slugger Field and Waterfront Park.
The other victims included Deana Eckert, 57; Jim Tutt, 64; Joshua Barrick, 40; and Juliana Farmer, 57.
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, Tennessee, about 160 miles (260 kilometers) to the south.
At least nine people, three in critical condition, were transported to the University of Louisville Hospital, including one officer, police said.
Police responded to reports of a mass shooting near Old National Bank in the centre of the southern city, an area frequented by tourist and residents alike.
The gunman, who also died from gunshot wounds, has been identified as Connor Sturgeon, 23, a bank employee.
The deputy chief said the quick action undertaken by police officers 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.”
In an emotional address, Kentucky's Governor Andy Beshear asked for prayers and said he did business at the bank involved.
“This is awful,” said Mr Beshear. “I have a very close friend that didn't make it today and I have another close friend who did not either and one who is at the hospital that I hope is going to make it through.”
The governor's voice cracked as he thanked law enforcement for “trying to save some of my friends and many others”.
The shooting in Louisville was the 146th mass shooting in the US this year, according to data from the Gun Violence Archive, which classifies a mass shooting as an incident in which four or more people — not including the gunman — are shot or killed.
President Joe Biden issued a strongly worded statement in response to the incident.
“Once again, our nation is in mourning after a senseless act of gun violence,” Mr Biden said.
The Democratic leader reiterated his call for gun reform.
“How many more Americans must die before Republicans in Congress will act to protect our communities?” he asked.
“It’s long past time that we require safe storage of firearms. Require background checks for all gun sales. Eliminate gun manufacturers’ immunity from liability. We can and must do these things now.”
Some of the state's most prominent politicians have reacted to the tragedy as well.
“Elaine and I are devastated by the news coming out of Louisville this morning,” said Senator Mitch McConnell on Twitter.
Senator Rand Paul said: “Our hearts break for the families of those lost.”
Agencies contributed to this report
