The green Humvee rolled up to the mangled railroad line on the northern edge of this city Tuesday, where thousands of U.S. and Iraqi forces had launched an offensive more than a week before in a shower of bullets and mortar shells. Pen and notepad in hand, Equipment Operator 1st Class William Seado of the U.S. Navy jumped out and started to inspect the tracks.

As he walked around a tanker car, Seado, 31, of Custer, S.D., noticed a black wire strung across the metal rails. He followed it to the tanker, where he found two sandbags filled with mortar rounds. Seado, a member of the Navy's elite Seabee Engineer Reconnaissance Team, raced back to the Humvee, which was parked only a few feet from the tanker -- well within what is called the kill zone of the improvised explosive device, or IED.

"Let's get out of here," he said, as he cranked up the engine and sped off. "There's an IED on the track."

Navy Lt. Jeffrey McCoy, the convoy commander who was sitting in the passenger seat, grabbed the radio handset to warn the other team members.

"I've never seen you move that fast, Seado," said McCoy, 31, of Youngstown, Ohio.

"I intend to get out of here with my butt in one piece," Seado replied, his voice flat and matter-of-fact.

The assault on Fallujah that began the night of Nov. 8 was aimed at breaking the insurgents' grip on a city they had controlled since April. It was the biggest military operation in Iraq since the invasion in March 2003, involving armored vehicles, artillery, airpower and thousands of troops. It was accompanied by a pledge from U.S. officials to rebuild the city after the offensive was complete.

But keeping that promise -- in a city rigged with booby-traps and explosives, with insurgents still fighting back in some neighborhoods -- may prove more difficult than anticipated, as the Seabees discovered on Tuesday. Their mission was supposed to be fairly simple -- get in, take some measurements, snap a few pictures, get out. That didn't happen.

After Seado discovered the bomb on the tanker car, the convoy quickly but gingerly backtracked across a muddy field dotted with land mines. Then the team spent nearly two hours securing the area around the rigged tanker before moving to another section of damaged railway. There, the Seabees spied the blue wire of a second bomb, forcing another quick exit. At a third stop, a group of Marines advised them to park behind a dirt berm because snipers were firing from a bank of houses a short distance away. The vehicles rolled over the berm and onto a flat area where construction stakes marked another batch of mines.

"These insurgents are really making things inconvenient," McCoy said.

As in other parts of Fallujah where U.S. forces have battled insurgents, the railway was more extensively damaged than the Seabees had expected. "It's pretty bad," Seado said. "I was hoping it wouldn't be that bad, but it's going to be a lot of work to rebuild."

McCoy said most of the city's basic infrastructure was damaged, not only by fighting but also by years of neglect.

"There's a lot of structural damage to houses and public buildings," he said. "The main utilities -- electricity, water, sewage -- are all going to need a lot of work."

On a tour of the city on Tuesday, Army Gen. George W. Casey Jr., the commander of U.S. forces in Iraq, passed rows of buildings riddled by bullets, tank rounds and artillery fire. Afterward, he said: "I must say, I didn't see a lot of major structural damage. There's a lot of debris in the streets. There's some electrical work that needs to be done, and there's some peripheral damage to the buildings, but I didn't see a lot of structural major damage, so I think it'll be a few months. Things will move along quickly once we get started."

Military engineers said they planned to begin making repairs to the city's infrastructure as soon as Fallujah was secure. But McCoy said those first repairs would amount to first aid, and that making Fallujah livable for its 250,000 residents, most of whom fled before the military operation, could take up to a year.

"If you leave them a mess like this with no running water, living in sewage, they are just going to be disgruntled," McCoy said. "We're going to have anti-American sentiment that's just going to breed. Reconstruction is as essential as the actual purging of the insurgents."

As the Seabees worked on Tuesday, it was clear that the insurgents had not been purged just yet. Heavy machine-gun fire thundered from the neighborhood next to the railroad tracks, and large explosions erupted close enough to make the ground shake and the Seabees scramble for cover.

Later, as he and McCoy rode away, Seado said he was disappointed they had not stuck around long enough for the special ordnance team to arrive. "I kind of wanted to see them blow up the tank car," Seado said.

"Why?" McCoy asked. "It's just another spot of damage we'll have to fix."

At left, Jeffrey McCoy, 31, of Youngstown, Ohio, watches for insurgents near railroad tracks in Fallujah. His crew of engineers cleared out minutes later after finding a detonation wire strung along the railway bed. At right, the Seabees discuss the damage to the track, which was bombed by U.S. warplanes to clear a path for troops going into the city during last week's massive offensive.