A US Navy sailor shot and wounded three people at the historic military base at Pearl Harbor, Hawaii, on Wednesday, military officials said, before fatally shooting himself.
Joint Base Pearl Harbor-Hickam said on Twitter that the three injured victims were all civilians working for the US Department of Defense, but gave no information on their condition.
NBC affiliate Hawaii News Now reported that two of the wounded were in critical condition at local hospitals.
"I join in solidarity with the people of Hawaii as we express our heartbreak over this tragedy and concern for those affected by the shooting," governor David Ige said on Twitter. "Details are still emerging as security forces at Joint Base Pearl Harbor Hickam investigate."
Mr Ige said the White House had contacted him to offer assistance from federal agencies as needed.
"The president has been briefed on the shooting at Joint Base Pearl Harbor-Hickam in Hawaii and continues to monitor the situation," a White House spokesman said.
The base, which was formed by the merger of Pearl Harbor Naval Station and Hickam Air Force Base, was placed on lockdown for about two hours following the incident at about 2.30pm Hawaii standard time.
"Base security and Navy investigative services are currently investigating. The names of the victims will not be released until the next of kin have been notified," Joint Base Pearl Harbor-Hickam said in a tweet.
An unnamed witness told Hawaii News Now in an on-air interview that he had heard gunfire near Drydock 2 at the base and looked up from his desk to see the gunman put a gun to his head and shoot himself. The gunman was wearing a US Navy uniform, the witness told the station.
Hawaii News Now reported that two of the victims were in grave condition at local hospitals and that the third was listed as "guarded."
Joint Base Pearl Harbor-Hickam is a combined US Air Force and Navy Installation located 13km from Honolulu.
The incident comes three days before the 78th anniversary of the Dec 7, 1941 attack on the naval base that led the United States to enter World War Two by declaring war on Japan.