A disgruntled city employee opened fire at the municipal centre in Virginia Beach, Virginia, killing 12 people before he was killed himself, the city's police chief said.
Virginia Beach police chief James Cervera gave few details of what was known about the mass shooting in the coastal resort city, but said the gunman "immediately and indiscriminately fired upon all the victims" after he entered the municipal building shortly after 4pm.
"The suspect did shoot a police officer. The officers returned fire. The suspect is deceased," Mr Cervera said. A city spokesman later confirmed to Reuters by email that the gunman was shot by police and later pronounced dead at a hospital.
The officer hit was among six people who survived their injuries, and he was saved by his bullet-proof vest, Mr Cervera said.
The shooting was believed to be the deadliest act of workplace gun violence in the United States since February, when a factory worker shot five colleagues to death in Aurora, Illinois, just after he was let go from his job.
The precise circumstances of Friday's shooting remained under investigation, with FBI agents and Homeland Security Department forensic technicians assisting local police due to the "size and scope and intensity" of the crime scene, Mr Cervera said.
The police chief said the suspect was a longtime public utilities employee, and described him as "disgruntled," but declined to say more about what may have precipitated the attack.
"We have more questions than we really have answers," he told reporters about two hours after the shooting.
The bloodshed unfolded, according to Mr Cervera, at Building Two of the municipal centre complex, a facility which houses the city's public works and utilities next door to City Hall.
The complex lies several miles inland from the town's popular seashore, situated on the Atlantic coast at the mouth of the Chesapeake Bay. It ranks as Virginia's most populous city with roughly 450,000 year-round residents.
"This is the most devastating day in the history of Virginia Beach," Mayor Bobby Dyer said at a news conference with the police chief. "The people involved are our friends, co-workers, neighbors, colleagues."
Arriving in town a short time after the carnage, Virginia Governor Ralph Northam said. "This is just a horrific day."
In Washington, a White House spokesman said President Donald Trump had been briefed on the shooting and "continues to monitor the situation."