Republican leader and Kentucky Sen. Mitch McConnell, who lives in Louisville, tweeted that he's "devastated" by the news of Monday morning's mass shooting.

"We send our prayers to the victims, their families, and the city of Louisville as we await more information," McConnell tweeted.

His fellow Kentucky senator, Rand Paul, tweeted, "Our hearts break for the families of those lost."

President Joe Biden has been briefed on the shooting, a White House official confirmed to ABC News.

-ABC News' Allie Pecorin and Justin Gomez

Officers arrived at the Old National Bank within three minutes of being dispatched and found the suspect still firing, Louisville police said.

Officers exchanged gunfire with the suspect who died at the scene, police said. It's not yet clear if the suspect's wounds were self-inflicted, police said.

Four victims were killed and eight people are injured, police said.

At least two officers were shot during the exchange of gunfire, police said, including one who is undergoing surgery.

It appears the suspect was a previous or current employee, police said.

"This is awful. I have a very close friend that didn't make it today. And I have another close friend who didn't, either, and one who is at the hospital that I hope is gonna make it through," Kentucky Gov. Andy Beshear said at a news conference.

"The safety of Old National Bank employees and everyone we serve in our banking center locations is paramount,” Jim Ryan, the CEO of Old National Bank, said in a statement. “As we await more details, we are deploying employee assistance support and keeping everyone affected by this tragedy in our thoughts and prayers."

There have been at least 15 mass shootings in the U.S. in the first 10 days of April, including Monday morning's shooting in Louisville, according to the Gun Violence Archive.

So far this year, the nation has seen at least 146 mass shootings.

The Gun Violence Archive defines a mass shooting as at least four people shot, not including the suspect.

“It feels like every day in this country we are totally consumed by yet another mass shooting. Nowhere else in the developed world do people wake up to this preventable horror every single morning," Kris Brown, president of the Brady Center to Prevent Gun Violence, said in a statement Monday. "Whether it's a bank, a school, a supermarket, or a church, Americans no longer feel safe in their communities. And Americans are increasingly tired of living in fear of being a victim of a mass shooting."

“It does not have to be this way," Brown said. "But until the gun industry no longer has a vice grip on our elected officials, this will continue to be our daily reality."

-ABC News' Ahmad Hemingway

The gunman opened fire in the Old National Bank's first-floor conference room at about 8:30 a.m. local time, according to officials.

Eyewitnesses said the shooter appeared to have been armed with a long gun.

"He just started shooting," Troy Haste told ABC Louisville affiliate WHAS. "Whoever was next to me got shot. Blood is on me from it."

Police said the suspect is dead.

A responding police officer was shot in the head, sources told ABC News. The officer’s condition is unknown.

According to preliminary information, this shooting is not believed to have been terror-related, sources told ABC News.

-ABC News' Jack Date, Josh Margolin, Luke Barr and Aaron Katersky

Louisville police have released body camera footage from Monday's mass shooting.

The footage shows the interaction between the shooter and responding officers, including Nickolas Wilt, who was shot and remains in critical condition.

"You will see he never hesitates," Deputy Chief Paul Humphrey said of Wilt during a press briefing on the footage. "This young man went back in to the line of fire."

Humphrey said the suspect "lied in wait" for officers to respond to the bank shooting.

The officers could not see inside the building on their approach, and the footage conveys the "tension" of the scene, Humphrey said.

Officers and EMS treated victims at the scene of the shooting. The actions of first responders "absolutely saved lives," Humphrey said.

Louisville Mayor Craig Greenberg said authorities plan to release 911 audio of the incident within the next 24 hours.

"We know there are still a lot of unanswered questions," Greenberg said at the briefing. "We will continue to provide information as soon as it is available."