A 23-year-old bank employee with a rifle shot and killed four people and wounded nine others at his Louisville, Kentucky, work site on Monday morning and then was killed by police, authorities said.

Gunshots were heard around 8:38 am at the Old National Bank before it opened. Police arrived at the scene within three minutes and exchanged fire with the gunman, Louisville interim Metro Police Department Chief Jacquelyn Gwinn-Villaroel said at a news conference. "We then returned fire and stopped that threat," she said.

The gunman was identified as Connor Sturgeon, a 23-year-old white male. The gun used in the shooting was an AR-15-style rifle, a federal law enforcement source told CNN.

Sturgeon was notified that he was going to be fired from the bank, and he wrote a note for his parents and a friend indicating that he was going to shoot at the bank, said a law enforcement source familiar with the investigation, according to CNN. Sources indicated that he had issues of low self-esteem growing up. 

The chief said Sturgeon was livestreaming during the attack. "That's tragic to know that that incident was out there and captured," she said. "We're hopeful that we can have that incident removed, that footage removed." 

Meta, the company that owns Facebook and Instagram, said in a statement that it had "quickly removed the livestream of this tragic incident this morning".

Nine people, including two police officers, were treated for injuries from the shooting, University of Louisville Hospital spokeswoman Heather Fountaine said in an email. 

Officer Nicolas Wilt, 26, was shot in the head during an exchange of fire with the gunman, police said. He is a recent graduate from the police academy, sworn in as an officer less than two weeks ago on March 31. He was out of surgery and listed in stable but critical condition. 

Police released the names of the four people who died: Tommy Elliott, 63; Jim Tutt, 64; Josh Barrick, 40; and Juliana Farmer, 57.

An emotional Kentucky Governor Andy Beshear told a news conference that Elliott was an "incredible friend". He said Elliott helped him to build his law career, to win the governor office, and offered him advice on "being a good dad". 

Another close friend of Beshear's was wounded and in critical condition. 

Beshear praised the police for their quick response and said that there was no doubt that officers saved lives. 

"Our bodies and minds are not meant to go through this type of tragedies," said Beshear in the morning news conference.

The shooting, the 15th mass killing in the US this year, came two weeks after a former student killed three children and three adults at a Christian elementary school in Nashville, Tennessee. That state's governor and his wife also had friends killed in that shooting. 

Louisville Mayor Craig Greenberg, himself a workplace-shooting survivor, called the shooting "an evil act" and "targeted violence". 

"To add to this tragedy, a few blocks away, shortly after this tragedy, another man lost his life, and a woman was shot in a completely different act of targeted violence," said Greenberg. 

The incidents appeared to be unrelated, but they both took lives, said Greenberg. "They both leave people scarred, grieving and angry. I share all those feelings myself right now."

President Joe Biden responded to the shooting in a tweet: "Once again, our nation mourns after a senseless act of gun violence — Jill and I pray for the lives lost and impacted by today' shooting. Too many Americans are paying for the price of inaction with their lives. When will Republicans in Congress act to protect our communities?"

On Monday, police in Nashville disclosed that while they haven't discovered a motive in that shooting, writings left behind by Audrey Hale indicated that she had planned a mass shooting for months. Hale was not on the police's radar before the shooting.

Police also said Hale was under a doctor's care for an undisclosed "emotional disorder", but didn't say if there is a link between the disorder and the shooting.

Agencies contributed to this story.