CNN– A gunman opened fire at a bank in downtown Louisville, Kentucky, on Monday morning, killing four people and sending at least eight to the hospital before he was killed at the scene, Louisville Metro Police Deputy Chief Paul Humphrey said.

The shooter appears to be a former employee of the bank, he said.

“We believe this is a lone gunman who did have a connection to the bank,” he said.

The mass shooting began at about 8:30 a.m. Monday at the Old National Bank on East Main Street, police said.

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Officers responded to the scene within three minutes of being dispatched and found the shooter still firing, he said. They exchanged gunfire with the suspect, who died at the scene, police said. It was unclear if he was killed by an officer or died of a self-inflicted gunshot, he said.

Four people were found dead inside, and eight people are being treated at the hospital, including at least two officers who were shot, he said. One of those officers is in critical condition.

Mayor Craig Greenberg thanked the “brave and heroic” first responders for their actions. “Without a doubt their actions saved lives,” he said.

A large police presence in the area and as well as shattered glass at the entrance of the Old National Bank is seen in video by CNN affiliate WDRB.

Prior to the bank shooting, there had been at least 145 mass shootings in the US this year, according to data from the Gun Violence Archive. The Gun Violence Archive, like CNN, defines a mass shooting as one in which at least four people are shot, excluding the shooter.

Witness says he saw people running

Caleb Goodlett got a call from his wife, who works at Old National Bank, he told CNN affiiliate WLKY. She told him there was an active shooter and she was locked in a vault inside the bank. Goodlett called 911 but officers were already aware of the shooting.

He described it as a traumatic phone call at 8:30 a.m., and said he has since talked to and seen his wife and that she is fine.

Hagan Curd was walking his dog on East Main Street on Monday morning when he began to see people running in his direction, he said. Several gunshots can be heard in video he took at the scene.

“I saw people running and then some others got out (of) their car and began to run as well,” Curd said. “Then I heard police yelling and telling people there was a shooter at the bank and to back away.”

The Old National Bank in Louisville lists its opening hours as 9 a.m. to 5 p.m., meaning the shooting took place before it was open. The bank is dual headquartered in Evansville, Indiana, and Chicago and has more than 250 banking centers across the Midwest.

Kentucky Gov. Andy Beshear said he has two close friends who “didn’t make it today” and another at the hospital “that I hope is going to make it through.” He said his state attorney general campaign was run out of that building.

“I know virtually everyone in it, that’s my bank,” he said.

The Louisville metro area, located along Kentucky’s border with Indiana, has a population of about 630,000 people as of the 2020 Census. The 300 block of East Main Street is bordered to the east by the minor league stadium Louisville Slugger Field and includes a dentist’s office, a furniture store and the Louisville Ballet.

This is a developing story and will be updated.

CNN’s John Miller and Celina Tebor contributed to this report.