From my understanding of this incident (what I read in Stars & Stripes yesterday), this all took place at what we call a "Combat Outpost (COP)" or a patrol base in the Army. An example of a COP would be a factory or other complex in a neighborhood where a unit is conducting a series of operations - the unit temporarily uses a civilian facility as a base of operations and then later moves out of the area.
It sounds to me like the Marines moved into this type of camp, swept the area for mines, IEDs, unexploded ordnance, etc and had been there for a couple days when they held the promotion ceremony (as lots of people do around the 1st of the month). Right after the ceremony, one of the Marines may have stepped on / triggered an IED that had been emplaced in the ground before they ever got there, and that they had somehow missed when they initially secured the place. That's all I'm going to speculate on - there's no way to say at this point whether it was placed there by someone who knew the Marines were coming there or not, but it's doubtful it was emplaced while they were actually there. Insurgents often booby-trap buildings in areas that they know U.S. and Iraqi forces are operating.
So it's not that the place wasn't secured, as in they were providing security for the promotion ceremony, it's just that somehow the IED was missed when they moved into the place.
Here's the article from Stars & Stripes:
Marines revise account of lethal IED blast
10 dead were in combat outpost
BY JOSEPH GIORDONO
Stars and Stripes
The 10 Marines killed in an explosion near Fallujahwere inside amakeshift combat outpost at the time of the attack and not on a foot patrol as originally reported, U.S. military officials confirmed Tuesday.
According to the new details, the Marines were finishing up a promotion ceremony in the patrol base when the blast occurred.
“It is suspected that one of the Marines triggered a hidden pressure plate initiation device, causing the explosion,” according to a Marine Corps news release. “Explosive experts believe fourartillery shells were buried in two separate locations.”
Eleven other Marines were wounded in the Thursday’s incident; seven of those Marines returned to duty shortly after the attack.
The unit had taken over an abandoned flour factory near Fallujah and had been using it as a makeshift base, officials said.
“The platoon swept the area for explosives and established security around the factory,” the Marine release read.
On the day of the attack, their company commander had traveled to the factory to promote three Marines in the field. As the ceremony finished, the assembled Marines were dismissed and milling about. It was then that the device was triggered, officials said.
All of the Marines were in full combat gear—including body armor—at the time of the incident.
The Marines were from Company F, 2nd Battalion, 7th Marine Regiment from Twentynine Palms, Calif. At least 75 Marines stationed at Twentynine Palms have been killed since the 2003 invasion, according to Pentagon records.
First reports by the military said the Marines had been on a foot patrol at the time of the attack. In the ensuing days, a militant group calling itself the Islamic Army of Iraq posted a video on the Internet, claiming it was a recording of the attack. That video showed a massive explosion hitting a group of what appeared to be U.S. troops flanking a Humvee, patrolling a street.
On Monday, U.S. military officials said the details of the Fallujah attack and the Internet video did not match; they did not offer the new details until Tuesday.
This week in hometowns across the nation, families and friends mourned the Marines. In California, Gov. Arnold Schwarzenegger ordered flags to fly at half-staff in their memory.