Fallujah, Iraq ? Eight months after U.S. and Iraqi troops killed an estimated 1,000 insurgents here in a battle that also cost more than 70 American lives, intelligence suggests that insurgents are filtering back into the former capital of the Iraqi resistance.

American commanders in Baghdad and Fallujah insist they control the city so tightly that the guerrillas cannot succeed in regaining a foothold. But they acknowledge that Fallujah remains a powerful icon to an insurgency that is keen to stop Sunni Arabs in Iraq’s western Al Anbar province from participating in an October referendum on Iraq’s proposed constitution.

“In their minds, I think it’s got significance because a lot of insurgents were killed there,” Lt. Gen. John R. Vines, the top ground commander in Iraq, said from military headquarters in Baghdad.

“This was a resounding defeat for them,” added Brig. Gen. Peter Vangjel, who oversees analysis of operations here, “and they want it back.”

The prospect of insurgents infiltrating the city presents a daunting problem for U.S. military officials. For the embryonic Iraqi government as well as the U.S.-led coalition, commanders say, what happens in Fallujah will symbolize the success or failure of the war.

If insurgents succeed in returning, they would be rolling back the U.S.-led coalition’s largest and most successful military victory since the fall of Baghdad in April 2003.

The fact that the Marines are allowing former Fallujah residents to return only adds to the concern. So far, 140,000 of the city’s 250,000 residents have come back.

As Marines continue to relax restrictions on Fallujah entry points, intelligence leads suggest that insurgents who have already entered the city and others who might soon return have continued to plan attacks against Americans.

Marine Maj. Gen. Stephen T. Johnson, commander of coalition forces in western Iraq, said it’s not unusual that insurgents would want to return to Fallujah.

There is little evidence in the street so far of the insurgents’ return. U.S. troops who took the city center door by door late last year now roll through in the beds of open-backed Humvees. One group stopped to walk the streets as a visibly Western reporter questioned Iraqis, a liberty unavailable in other major cities in Iraq’s perilous Sunni Triangle.

The country’s precarious security situation and bare-bones government infrastructure have left the U.S. diplomatic mission largely unengaged in Al Anbar province at large. As American diplomats mill busily about their embassy offices in Saddam Hussein’s former Republican Palace in Baghdad’s cloistered Green Zone, 33-year-old John Kael Weston is the lone representative of the State Department paired with the 28,000 U.S. troops in the massive swath of desert stretching from Fallujah to the western borders with Jordan and Syria.

“With the normal citizens coming back, you’re going to have some insurgents too. They ran this city. I don’t think they’re going to forget what a safe haven they had,” Weston said. “If Fallujah turns into a Green Zone for bad guys again, then what will all this mean?”