A disgruntled ex-worker opened fire in Old National Bank in Louisville, Kentucky, killing four innocent people and wounding nine, before dying from a gunshot themselves.
The Louisville Metropolitan Police Department confirmed that officers responded to "reports of an active aggressor" after the gunman opened fire inside the bank.
The University of Louisville Hospital said it received nine patients including seven civilians and two police officers.
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Two officers were shot during the exchange, Louisville Metro Police Deputy Chief Paul Humphrey said.
Of the injured, two are in critical condition, one of which is a police officer.
At least three of the patients have already been discharged, according to the hospital.
Sources said armed police had to force their way into the bank, where terrified workers had taken shelter inside a vault.
Kentucky Governor Andy Beshear revealed he knew some of the Louisville bank victims personally.
Holding back tears, he said at a press conference: “This is awful. I have a very close friend who didn’t make it today. And I have another close friend who didn’t, either. And one who’s at the hospital that I hope is going to make it through.”
Mr Humphrey called the shooting "a tragic event” but praised officers who prevented more victims.
He revealed: "It was the heroic response of officers that made sure that no more people were more seriously injured than what happened.”
A witness inside the building told WHAS11 they saw a man with a "long assault rifle" fire multiple shots inside the bank on the first floor.
The eyewitness said: "He just started firing. I didn't see his face. We were in the conference room. Whoever was next to me got shot, there's blood on me."
Old National Bank CEO Jim Ryan was en route to Louisville to help assist staff affected by the tragic shooting.
He said: “The safety of Old National Bank employees and everyone we serve in our banking centre locations is paramount.
“As we await more details, we are deploying employee assistance support and keeping everyone affected by this tragedy in our thoughts and prayers.”
President Joe Biden paid tribute to the victims as he once again called for gun reform.
He tweeted: "Once again, our nation mourns after a senseless act of gun violence – Jill and I pray for the lives lost and impacted by today's shooting.Too many Americans are paying for the price of inaction with their lives.When will Republicans in Congress act to protect our communities?"
House Speaker Nancy Pelosi took to Twitter to comment on the shooting, calling it a "tragedy". She said: "The tragedy in Lousville, Kentucky, is the 15th mass shooting in the first 10 days of April. Our hearts break for all affected.
"Too many families are grieving loss - but we are not helpless. We can act; we must act to End Gun Violence."
Republican Morgan McGarvey called the shooting a "dark day" and revealed he is "heartbroken" by the violence.
He tweeted: "Today is a dark day in our community.I'm heartbroken by the loss of at least four innocent Louisvillians in yet another act of senseless gun violence, this time in the heart of our city, and I'm grateful for the heroic acts of first responders who no doubt saved lives today."
It was initially believed the motive for the attack may not have been a robbery.
Mr Humphrey said the gunman “did have a connection to the bank,” believing him to be a former employee.
Armed officers responded within three minutes of their first call encountering the suspect “almost immediately," the police chief added.
The suspect was still firing gunshots, according to officers. It was unclear if the killer died from gunfire or a self-inflicted wound.
Mr Humphrey confirmed the shooter was a "lone gunman."
He said: "We're trying to establish what that connection was to the business but it appears he was a previous employee."
"We're trying to establish what that connection was to the business but it appears he was a previous employee.
Victims have been rushed to a nearby hospital, including a heroic police officer who suffered various injuries, after an eyewitness claimed they heard "10-15 shots and screaming" from within the building.
Terrance Sullivan, who lives in a nearby building, told Courier Journal: "It was a lot of activity for like 20 seconds.
"The sound is what I keep thinking about. It was so loud. People in my building who were inside could hear it. Being outside on the street as it happened - I've heard gunshots before, but not that many like that."
Michael Clemons, who lived near the shooter, revealed when he went for a run he was stunned to find three people outside his house crying.
He tweeted: "My block, where the alleged shooter used to live, is surrounded with cops now. When I ran by at 9:15 there was a cop car at the house and three people standing outside crying. This hit too close to home."
Mr Humphrey confirmed the suspect was dead on arrival and the active shooter threat is over.
He said: "The shooter was confirmed to be dead on the scene. We do not know exactly the circumstances of his death at this time."
He said there was no active danger to the public at this time.
However, members of the public being asked to stay away from the scene.
Bullet holes and shattered glass were spotted outside the building where the gunman opened fire on victims.
The suspect's vehicle, a Mazda 6, has been located outside the bank - with the FBI currently at the scene.
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