WASHINGTON (CNN) -- A Marine shot and killed two of his fellow service members at a Virginia base Thursday night and then apparently killed himself, base officials said.

The incident took place at Marine Corps Base Quantico. The shooter gunned down a man and a woman, the spokesmen said. All are Marines -- permanent personnel assigned to the officer candidate school.

Authorities did not disclose a motive and were investigating the incident. The identities of the victims were not immediately disclosed as authorities worked to notify next of kin.

"It's been a long night," Col David Maxwell, the base commander, said Friday.

The incident shocked the military community. It comes as the Corps grieves over another tragedy: the deaths of seven U.S. Marines in a training exercise in Nevada on Monday.

Secretary of Defense Chuck Hagel "was saddened to learn of the shootings at Marine Corps Base Quantico," Pentagon spokesman George Little said Friday.

"This tragedy, as well as the tragedy in Nevada earlier this week, took the lives of Marines who volunteered to serve their nation. 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," Little said.

Base lockdown

Early Marine accounts said the notification of the shooting came late Thursday when authorities found one dead victim.

Using a public address system, police announced a lockdown at the base and surrounded the building where the suspect was believed to be barricaded. They ordered everyone to stay in their homes, doors locked, they said.

Authorities from the base and Prince William County, Virginia, surrounded the shooter, who was in a barracks not far from where the first shooting occurred.

Law enforcement officials entered the barracks two hours later and discovered the shooter dead, apparently of a self-inflicted wound. The female victim was in the room with him, fatally shot.

The lockdown ended early Friday, a few hours after it started.

As dawn rose over the base, Maxwell told reporters that there were no barricades or a standoff but did not elaborate. The incident was confined to a single barracks at the candidate school.

He said "quick" police reaction helped resolve the situation, first reported at 10:30 p.m. via an emergency call. Soon, security was heightened and a mass notification was issued throughout the base. Base restrictions were lifted a few hours later.

Others in the officer candidate school were "accounted for and safe."

"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.

Quantico, a historic hothouse of Marine ideas and training

Visitors to the base will see a grand statue: the Iwo Jima Monument. It gives all who enter pause about the important role the base has played training generations of Marines in forging military doctrine and fighting America's wars.

The statue depicts the historic flag-raising on Mount Suribachi, on the Japanese island of Iwo Jima, during the famous World War II battle there.

The Quantico base, started during World War I, has been integral in training, education and forging military doctrine for America's battles.

Early on, amphibious warfare was, the corps said, "conceived and perfected" there. The Marines there came up with the idea "of carrying troops from ship to shore by helicopter."

Now, it's home to the Marine Corps Warfighting Laboratory, which conducts experiments in modern warfare.

"This is perhaps the only command whose mission touches the farthest reaches of the Corps; decisions made here impact Marines aboard ship, fighting in the Global War on Terrorism, on guard duty at embassies across the globe and reserve duty throughout the United States," the Marines said.

Federal agencies, such as the Drug Enforcement Administration and the FBI, share training facilities there.

The base also has a reputation across popular culture as a key national security training location. Jack Ryan, writer Tom Clancy's well-known character in novels and films, for example, took officer training there.

CNN's Jason Hanna and Jake Carpenter contributed to this report.