Updated

Down a steep staircase littered with glass shards and rubble, U.S. Marines descended Thursday to a dark basement believed to have been one of Fallujah's torture chambers.

They found bloodstains and a single bloody hand print on the wall — evidence of the horrors once carried out in this former insurgent stronghold.

"We had sensed that there was a pure streak of evil in this town, ever since the first days of engagement here," said Maj. Wade Weems.

The basement, discovered while Marines fought fierce battles with Fallujah insurgents last month, is part of the Islamic Resistance Center (search), a three-story building in the heart of this city 40 miles west of Baghdad.

Maj. Alex Ray, an operations officer with the 1st Marine Expeditionary Force (search), said all evidence indicates the 15-foot-by-20-foot space was used by insurgents to imprison and torture their captives.

"Based on the evidence we have found here, we believe people were held here and possibly tortured — we have found enough blood to surmise that," Ray told reporters shown the basement Thursday.

On the wall adjacent to the hand print, human fingernails were found dug deep into the porous gravel around a hole in the wall — evidence, the Marines say, of a tunnel-digging attempt.

Although most of the evidence had been taken away, there was enough to suggest "they tried to dig their way out," Ray said.

No bodies or human remains — except for the fingernails — were found when the Marines discovered the underground chamber on Nov. 11, but they found "plenty of blood," he said. Marine experts have collected samples for forensic and DNA testing.

"This is tangible proof how horrific they were," Weems, of Washington, D.C., said of the insurgents, shuddering as he gazed at the bloody hand print.

Although unmarked, the center was a known base of operations for the insurgents who ruled Fallujah with terror and fear until U.S. forces and Iraqi troops captured it last month.

The assault was launched Nov. 8 to wrest Fallujah from the control of radical clerics and fighters who seized it after the Marines lifted a three-week siege of the city in April. The city fell after a week of fierce battles and overpowering airstrikes which reduced many of the buildings to rubble.

Two weeks later, Marines continue to fight sporadic gunbattles with holdouts as they clear streets, homes and buildings of weapons caches and rubble. More than 350 weapons caches have been found so far.

As Weems' troops inspected the Islamic Resistance Center on Thursday, gunshots and small arms fire reverberated from Fallujah's northeastern Askari (search) neighborhood. The Marines said it was a sign the insurgents are still active.

On the Islamic center's first floor, the Marines discovered a weapons-making factory at the back of what appeared to be a legitimate computer store.

It contained boxloads of empty shotgun shells and a primitive-looking reloading machine on one of the tables. On the second floor, they found a sack of gunpowder and numerous mortar launcher cases.

Elsewhere in Fallujah, the Marines have discovered DVD recordings of beheadings, as well as a cage and chains bearing traces of human blood. They say it was "apparent the cage was not holding animals."

"It's the combination of the chains, the cage, the blood — there were not nice people here, that's for sure," Ray said. "They certainly didn't have the morals I would expect in a human society."

Reporters were not taken Thursday to the other sites, many of which have been cleared of evidence and the buildings destroyed by the Marines.

Maj. Jim West, a Marine intelligence officer, has said Fallujah's "atrocity sites" were used by the insurgents to imprison, torture and kill hostages. In some, knives and black hoods, many of them blood-covered, have been found.

More than 30 foreign hostages have been killed by their captors in Iraq this year, including three Americans. Many of the victims have been beheaded and their deaths shown on grisly videos posted on the Internet. Iraqi police and other security forces have also been killed after their capture by insurgents.

"We believe the majority of the hostages were held in Fallujah because it was such an insurgent haven," said Ray.

The military says an estimated 1,200 insurgents and more than 50 Marines have been killed in the assault on Fallujah.