A Marine opened fire on two of his comrades Thursday night at a base in Quantico, Va., before turning the gun on himself, leaving all three dead, military officials said.

A relationship dispute was believed to be behind the shooting, which occurred in the staff barracks area of the Officer Candidates School at Marine Corps Base Quantico in Northern Virginia, a Marine official said.

The assailant and both victims, a man and a woman, were staff members at the school and not students, a senior defense official said. The official called the incident "isolated," adding: "There was nothing random here."

The names of the dead were being withheld for 24 hours pending notification of next of kin.

Military police and Prince William County, Va., Sheriff's Department officers responded to a report of gunshots around 10:30 p.m. local time and were on the scene within five minutes, base commander Col. David W. Maxwell said at a press conference on Friday.

They found one Marine dead and the shooter inside the barracks, a Marine official said. At 3 a.m., officers entered the barracks and found two more bodies, including that of the shooter, the official said. The three, all active duty Marines, were pronounced dead at the scene.

Why officers hesitated before entering the barracks remained unclear Friday morning.

“There was a lag but I can’t go into the details about the length of the lag the occurred,” base spokesman Lt. Agustin Solivan said Friday.

The assailant appeared to have died from a self-inflicted gunshot wound, base officials said.

Authorities did not fire any shots and did not hear any shots fired, Maxwell said. “There was no standoff or barricades,” Maxwell said.

Early reports indicated that the shooter may have barricaded himself in the barracks.

The base had been under lockdown after the incident, but its status early Friday returned to "Code Green," meaning operations were normal.

A message posted on Quantico's Facebook page had earlier had told residents to remain in their homes with their doors locked. Personnel on the base were notified of the situation via an emergency alert system. The base returned to normal operations at 2:30 a.m., Maxwell said.

“Early this morning, the Secretary was saddened to learn of the shootings at Marine Corps Base Quantico,” defense department spokesman George Little said in a written statement. “His heart and his prayers are with them and their families.”

Quantico is about 40 miles south of Washington in the Northern Virginia suburbs.

“This is a truly tragic loss again for the Marine Corps, which has had a number of tragic losses in the last couple of weeks,” Maxwell said.

The base provided chaplains and counselors to base residents on Friday.

The Officer Candidates School calls itself "the first proving ground for future Marine officers." Its graduates attain the rank of 2nd lieutenant.

It has a reputation for being challenging.

"The mission of OCS is to train, screen and evaluate candidates, who must demonstrate a high level of leadership potential and commitment to success in order to earn a commission," the Marine Corps says on the school's website. "Officer Candidates School training will be more demanding than any you've experienced before, regardless of commissioning program."

Matthew Barakat / AP

The entrance to Marine Corps Base Quantico in Northern Virginia is shown early Friday after three Marines, including the suspected assailant, died in a shooting.

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NBC News' Denise Ono, Christopher Nelson and Matthew DeLuca contributed to this report.

The Army said Monday that an unborn child was not among the 17 victims in the shooting massacre of civilians in two villages in Afghanistan allegedly perpetrated by Army Staff Sgt. Robert Bales, contradicting an Afghan official who spoke to The New York Times.

Kandahar Province Police Chief Brig. Gen. Abdul Raziq told The New York Times that one of the slain females was pregnant and Americans were counting her unborn fetus as a victim. But Army Lt. Col. Jimmie E. Cummings, Jr., told msnbc.com a fetus was not among the victims of the March 11 attacks in which Bales, 38, is charged with premeditated murder.

“The information that we have collected up to now, this is not true,” Cummings, a spokesman for NATO's ISAF & U.S. Forces - Afghanistan, wrote in an email to msnbc.com. “The 17th is not from a pregnant female or any of the wounded passing away. At this time, the evidence available to the prosecution team indicates 17 victims of premeditated murder and 6 victims of assault and attempted premeditated murder.” 

The death toll breaks down to four men and women each, and nine children, Cummings wrote. One man and one woman, plus four more children, were wounded.

“I think one of the things you can assume is that it was difficult to collect evidence in this case and it was difficult for them to necessarily identify every victim right away,” said Michael Navarre, a director of the National Institute of Military Justice and a former Navy prosecutor and defense counsel.

New details emerged over the weekend in the case. Military prosecutors told NBC News that the attacks came in two waves, with Bales allegedly returning to his base after the first attack and then slipping out again.

The father of two from Bellevue, Wash., was charged with 17 counts of premeditated murder, six counts of attempted murder and six counts of assault. He is being held at a U.S. military prison in Fort Leavenworth, Kan.

His wife, Karilyn Bales, said she did not believe her husband had done this.

“I don't think anything will really change my mind in believing that he did not do this,’’ she told TODAY’s Matt Lauer in an exclusive interview that aired Monday. “This is not what it appears to be.’’

Military wives rally around Karilyn Bales

“I just don't think he was involved,’’ she said. “I don't know enough information. This is not him. It's not him."

The timeline of the killings remains unclear. One Afghan guard working from midnight to 2 a.m. saw a U.S. soldier return at 1:30 a.m., and the guard’s replacement saw a U.S. soldier leaving the base at 2:30 a.m., but it was unclear whether it was the same soldier.

There are reports that there is surveillance video, and that Bales allegedly walked back to the base and turned himself in.

For alleged Afghan shooter, death penalty unlikely

Karilyn Bales said her husband was fit for a fourth deployment and that she was not aware of any obvious signs of post-traumatic stress disorder or the traumatic brain injury that he allegedly suffered on one of his tours. 

Bales was on his fourth tour in a war zone since signing up for the Army after the 9/11 terrorist attacks in 2001. He had spent three years in Iraq on his previous tours, during which time he lost part of a foot and suffered a traumatic brain injury (TBI) due to a vehicle rollover, media reports say. Two days before he allegedly attacked the Afghan villagers, he saw the aftermath of a bombing in which a fellow soldier had his leg blown off, The Associated Press reported.

Some military law experts interviewed by msnbc.com said they expect the defense to mount a legal pincer attack, in which Bales’ attorneys may try to win acquittal by attacking the evidence but have a fallback position aimed at winning a lesser sentence than the death penalty -- which Defense Secretary Leon Panetta has said could be sought in this case.

How Staff Sgt. Bales' lawyers are fighting for his life

Gary Solis, former head of the Marine Corps’ Military Law Branch and current adjunct professor of law at Georgetown Law School, said the fact that the crime occurred in a combat zone in a distant country complicates the task for prosecutors given the possibility of numerous crime scene complications. But they agreed that pursuing an insanity defense based on PTSD would be a difficult case to make, too.

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JOINT BASE LEWIS-MCCHORD, Wash. – A company of soldiers has been released from lockdown at Joint Base Lewis-McChord in Washington as criminal investigators probe the theft of $600,000 worth of weapons accessories.

About 100 soldiers from the 4th Stryker Brigade, 2nd Infantry Division were allowed to return home Tuesday evening after being restricted to their battalion headquarters since Jan. 4.

Sources told KING5 the equipment went missing sometime between Christmas and New Year's. Someone realized a lock had been changed in the weapons room, and no one had a key to it.

Read original story on KING5.com

After breaking through the lock, JBLM officials tallied about $600,000 worth of missing weapon sights, night vision devices, optics, rifle scopes and other items.

NBC sources said they do not believe the equipment was stolen for espionage, but rather to sell for profit.

Brigade commander Col. Michael Getchell told The Associated Press the restrictions "have been an integral part of the investigation."

An Associated Press phone message to the investigating office was not immediately returned.

Lockdowns are a common military practice in cases like these, JBLM spokesman Major Chris Ophardt told NWCN.com.

In March 2001, about 150 soldiers faced a similar situation over a one pair of missing night vision goggles, he said.

KING5 and The AP contributed to this report.

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