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 the dramatic 911 calls from Monday's mass shooting inside a bank.

One woman who called 911 said about eight or nine people have been shot. She said she was hiding in a closet in the first-floor conference room.

She told the dispatcher she knew the shooter, who worked with her.

Loud gunshots are heard booming in the background as the employee recounted what was happening.

The dispatcher told the woman to stay quiet and stay where she was. Whispering, she told the dispatcher there was a lot of blood.

The mother of the suspected gunman, 25-year-old Connor Sturgeon, called 911 to say her son's roommate called her and was concerned.

"He apparently left a note," she said.

"He's never hurt anyone, he's a really good kid," Sturgeon's mother told the dispatcher. "I don't know where he would've gotten a gun."

"I know he doesn't own any guns!" she told the dispatcher.

Sturgeon, who was killed by police at the scene, bought the AR-15 used in the attack legally on April 4 in Louisville, according to police.