At least three people have been shot in an "active shooter" situation at Pearl Harbor Naval Shipyard in Hawaii.
So far two people have critical injuries, with a third person with gunshot injuries also taken to hospital.
A witness said the gunman was wearing a naval uniform and shot several civilians.
A spokesman said the situation had been "contained", amid local media reports that the gunman was dead.
A witness said he was at his desk when he heard loud pops, Hawaii News Now reports.
"I kind of recognised that as gunshots," said the witness, "and I looked out the window and saw three people on the ground."
The gunman was wearing a sailor uniform, the witness said.
The base has been placed in lockdown and a PA system is urging people to take cover, Hawaii News Now reports.
Base personnel also received text messages alerting them of the situation.
Witnesses told Hawaii News Now that the shooting happened at Drydock 2.
First responders were called to the scene about 2:30pm local time.
A spokesman said the situation was still unfolding, but it wasn't immediately clear if a gunman was being sought.
The reported shooting occurred three days before commemorations of the Japanese attack that thrust the United States into World War II.
People at the scene say it is chaotic.