Four people were killed when an employee opened fire in a bank in Louisville, Kentucky, and livestreamed the attack on Instagram, police say.
The victims were aged between 40 and 64. Among nine others injured was a rookie police officer who graduated into the force two weeks ago.
The officer was struck in the head and is still critical after brain surgery.
Police responded within three minutes, and fatally shot the attacker in an exchange of gunfire.
The shooting took place at the Old National Bank in the city centre at about 08:30 local time (12:30 GMT).
Witness Caleb Goodlett told local media that his wife, an employee at the bank, had locked herself inside the vault when the attack began.
Other witnesses described seeing the shootout between police officers and the lone attacker.
Kentucky Governor Andy Beshear said an "incredible friend" of his, Tommy Elliot, a senior vice-president at the bank, was among the victims.
"Tommy Elliott helped me build my law career, helped me become governor, gave me advice on being a good dad," said Mr Beshear.
The victims have all been identified:
- Thomas Elliot, 63
- James Tutt, 64
- Joshua Barrick, 40
- Julianna Farmer, 45
Jim Ryan, CEO of the Old National Bank, said the safety of employees and customers was "paramount", adding that bank officials were "keeping everyone affected by this tragedy in our thoughts and prayers".
The officer who was struck in the head was identified as Louisville Metro police officer Nickolas Wilt, 26.
City Councilman Anthony Piagentini told the Courier-Journal newspaper that Mr Wilt graduated from the academy on 31 March, and that his brother is enrolled in the police academy.
Louisville Mayor Craig Greenberg called the attack "an evil act of targeted violence".
Two survivors told WHAS-TV that the gunshots first broke out in a ground-floor conference room where employees had gathered.
"Whoever was next to me got shot - blood is on me from it," said one man, pointing to his shirt.
The suspect was named as 23-year-old Connor Sturgeon, who police said used a rifle during the incident and was broadcasting the shooting online.
"That's tragic to know that that incident was out there and captured," said Police Chief Jacquelyn Gwinn-Villaroel.
Instagram's owner Meta said it had "quickly removed the livestream of this tragic incident this morning".
The attacker is described in an online profile as being a syndications associate and portfolio banker at Old National Bank. He had joined the company as a full-time employee last year after spending three summers as an intern there.
Officials say he had no previous contact with law enforcement.
The shooting took place just two weeks after a violent attack on a Christian school in Nashville, Tennessee - 160 miles (260km) - south of Louisville - that killed six people.
Data compiled by the Gun Violence Archive shows that there have been at least 146 mass shootings - defined as those in which at least four people were shot - so far in 2023, including at least 15 since April began.
Also on Monday morning, Louisville Police responded to a second shooting outside the Jefferson Community & Technical College, less than two miles away from the incident at Old National Bank.
At least one person was killed and one wounded, according to police. The two shootings are unrelated, LMPD Maj Russell Miller said.
President Joe Biden tweeted on Monday: "Too many Americans are paying for the price of inaction with their lives.
"When will Republicans in Congress act to protect our communities?"
Democrats and Mr Biden have long pushed for Congress to push stricter gun safety legislation. These efforts face staunch opposition from most Republican lawmakers, who often represent constituents and communities that support expanded gun rights.
Kentucky is one of 26 states that allow most adults over 21 years old to purchase and carry a firearm without a license.
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