FALLUJAH, Iraq – Down a steep staircase littered with glass shards and rubble, American Marines descended yesterday to a dark basement believed to have been one of Fallujah’s torture chambers. They found bloodstains and a single bloody handprint 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 Major Wade Weems.

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

Major Alex Ray, an operations officer with the 1st Marine Expeditionary Force, said all evidence indicates that 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,” Major Ray told reporters shown the basement yesterday.

On the wall adjacent to the handprint, human fingernails were found dug deep into the porous gravel around a hole in the wall – an indication, 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,” Major Ray said. No human remains – except for the fingernails – were found when the Marines discovered the underground chamber on November 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,” Major Weems, of Washington, D.C., said of the insurgents, shuddering as he gazed at the bloody handprint.

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

The assault was launched November 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 air strikes that reduced many of the buildings to rubble.

Two weeks later, Marines continue to fight sporadic gun battles 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 Major Weems’s troops inspected the Islamic Resistance Center yesterday, gunshots and small arms fire reverberated from Fallujah’s northeastern Askari 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 box loads 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.