The young Marine had just shot a suspected insurgent and was walking back across the villa's rooftop when he keeled over from a terrific thud to the back of his head.

A sniper had fired a single, well-aimed bullet that tore through the top of Lance Cpl. Richard Caseltine's helmet, traced a path along the edge of his skull and buried burning bullet fragments in the back of his neck.

Less than a minute later, the 20-year-old from Aurora, Ind., was up on his feet — crouching, shaking and miraculously, still alive.

"You expect when somebody gets shot in the head, they're dead," the soft-spoken Caseltine told The Associated Press in an interview, cradling the battered camouflage helmet that saved his life Saturday. "I consider myself very lucky."

Caseltine was among two squads from the 3rd Battalion, 8th Marine Regiment's India Company moving through the rocket-blasted streets of downtown Ramadi on a joint foot patrol with the Iraqi army.

Caseltine and several others were tasked with providing "overwatch" — finding a place from where they could watch over the rest of the patrol.

They entered the front gate of a two-story villa and herded a man, his wife and their children into a room.

Four Marines then climbed the stairs to a rooftop enclosed by shoulder-high walls, each taking positions in separate corners to scan adjacent buildings and streets.

Half an hour later, Lance Cpl. Benjamin Congleton, 22, of Lexington, Ky., spotted a man in a black T-shirt crouching on the ground near a light pole. He was fiddling with a tangle of wires and looking from side to side.

Congleton called Caseltine over for a second opinion. They agreed the man was trying to plant a bomb.

Congleton fired his M-16, but missed. The startled man tried to stand up. Caseltine fired his M-4 Carbine, hitting the man in the leg. Congleton then shot the man in the head as he tried to flee down an alleyway, apparently killing him.

Caseltine took three or four steps back to his position in the rooftop corner when he felt something strike the top backside of his helmet.

"It felt like somebody came from behind and punched me in the back of the head as hard as they could," Caseltine said. "It just rocked me. I went forward and my ears started ringing really bad. I couldn't hear anything."

It wasn't clear at first if one of the Marines had misfired one of their weapons. But in a split-second, they understood the sole shot had not come from them.

Ducking to the ground, they rushed to Caseltine's aid.

"He was yelling, 'I got hit! I got hit!' Congleton said.

A cursory check revealed blood at the back of Caseltine's neck but no serious wounds.

Caseltine was still conscious. Able to walk, he got up and, crouching, moved to the relative safety of a room downstairs, where a Navy medic examined him.

The back of his neck burned, but he was fine otherwise.

"He had this big smile on his face," said Lance Cpl. Jefferson Ortiz, 21, of Miami. "He knew he'd gotten very, very lucky."

As troops popped smoke grenades, a Humvee arrived to evacuated the wounded Marine.

Congleton said he believed the sniper had been providing "overwatch" for insurgents planting bombs in preparation for a major assault on the Marine-protected provincial government headquarters. The attack began the minute the rest of the squad exited the villa.

"We were taking fire from every street corner," Congleton said. "It seemed like we were fighting the entire city."

Bounding across rubble-strewn intersections nearby, one Iraqi soldier was hit by a bomb that blew other Iraqis into the air. Some got up and kept running, but one soldier lay writhing and bloodied — one of legs was partially detached. A couple Iraqi soldiers began dragging him by his clothes, but a Marine lifted the soldier onto his back and carried him away, Congleton said.

Caseltine, meanwhile, was flown to a military medical facility at nearby Balad air base, where medics removed fragments from the bullet that were lodged a quarter-inch into the back of his neck.

"They said I was lucky it didn't go in deeper. My luck was running pretty good that day," Caseltine said. "If I had bought a lottery ticket, I probably would have won."

On Tuesday, three days later, Caseltine was back on base, hours away from rejoining his squad at an outpost elsewhere in Ramadi. Sitting outside his sandbagged tent, he pulled out a photo that showed him cradling his wife. It had been ripped in two by the bullet — right down the middle.

Caseltine had stuffed it into the netting inside the top of his helmet, known as Kevlar for the protective material, "so my wife would be with me."

"They always tell us not to throw our Kevlars around or bang them on the ground. I usually do, but I ain't gonna' be throwing my new one down," Caseltine said. "I ain't gonna' take it for granted anymore because I know they work."