A Marine killed two fellow military staffers at Marine Corps Base Quantico in Virginia, then killed himself in what appeared to be an isolated incident, military officials said on Friday.

There was no immediate explanation for the motive of the gunman who shot a male and a female in a rampage that began Thursday night at the base, about 40 miles south of Washington. The three dead Marines were permanent personnel assigned to Officers Candidate School at the base.

“It’s been a long night,” Col. David W. Maxwell, the base commander, told reporters at a televised news conference on Friday.

Around 10:30 p.m. Thursday, officials found one Marine dead at the barracks, Maxwell said. A second Marine was found dead later, along with the body of the shooter, who apparently shot himself to death.

Earlier, officials had described the situation as a standoff, but corrected that Friday morning. The investigation of the incident is continuing.

None of the dead was identified, pending notification of their families.

The latest incident comes on the heels of a Nevada training exercise accident Monday night, in which a 60-millimeter mortar exploded, killing seven Marines and injuring eight.

In a statement, Pentagon press secretary George Little said Defense Secretary Chuck Hagel was saddened to learn of the Quantico shootings.

“This tragedy, as well as the tragedy in Nevada earlier this week, took the lives of Marines who volunteered to serve their nation,” Little said. “His heart and his prayers are with them and their families. He believes that the legendary strength of the United States Marine Corps will ensure that they are forever remembered.”

The Quantico base is also the site of the FBI’s training academy.

When the incident began, the base was locked down as a security precaution, but those restrictions were lifted Friday morning.

“As we take care of our Marines and their families that are dealing with this tragedy, I’d also ask for the support of our neighbors, the community, and their thoughts and prayers as well for our Marines who have lost their comrades-in-arms,” Maxwell said.

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