Mohamed, 28 — who grew up a tough kid of Egyptian, Russian and Irish descent from Rockaway, N.Y. — wore the boots when he went to war in Iraq after the terrorist attacks of Sept. 11, 2001.

On that clear day, he watched the Twin Towers fall on television while he was in English class at the Lindenhurst High School Alternative Learning Center in Long Island. On this day nearly a decade later, he was helping rebuild what al-Qaeda destroyed in an act he says determined the course of his life.

"You grow up here. 9/11 happens. You join (the National Guard). You go to war. You come home. And then you're rebuilding," he says. "You're like a full part of this whole thing."

About a thousand workers are building five office buildings on the World Trade Center site — including the 1,776-foot centerpiece, WTC 1. For some, the job has a particularly special meaning. Labor officials estimate that a few dozen or more U.S. military combat veterans of Iraq and Afghanistan, like Mohamed, are on construction crews working at the site.

They are here to restore what terrorists destroyed. An attack that prompted a U.S. invasion of Afghanistan and led these veterans-turned-construction workers to war and then back to the ground where 2,753 people died.

"It's not just a job. This is an opportunity for them to address the tragedy," says Anne Trenkle, New York state director for the Helmets to Hardhats program that directs veterans, reservists and National Guard members to local construction jobs. "They feel very strongly about (it)."

Mark Dudenhoffer, 29, was working on a garbage truck on 9/11 when Muslim jihadists seeking to humble American might slammed two hijacked passenger jets into the World Trade Center, turning it into "Ground Zero" 10 years ago. Later, he was among the workers called in to help recover the missing and the dead.

Dudenhoffer is the grandson of an ironworker who worked on the original towers when they were built in the early 1970s. He is also a New York National Guardsman who would ultimately fight in Iraq and Afghanistan.

"It's history," says Dudenhoffer, a combat buddy of Mohamed's and now a co-worker at the Trade Center site.

Another worker, John Paul "J.P." Cespedes, watched the black smoke billowing from the towers from an apartment building in Queens. A Marine by then, he would go to Iraq in 2003.

"You do get a sense of pride from the creation of a new tower," he says. "You see the end result and you think, 'Oh, yeah, I worked there. I helped there. I was there when they topped it out.'"

All three men are now on an 80-member ironworker, or latherers, crew at Tower 3, assembling metal bars, or rebars, to reinforce poured concrete in a building that will rise 80 stories.

"They were all good guys who served over there in the Middle East," says their crew boss, Billy Grogan.

The long-delayed project — ground wasn't broken on WTC 1 until 2006 — now has 1,000 workers broken into crews of ironworkers, glaziers, welders, mechanics, carpenters, masons and millworkers.

The five towers are at various stages of construction, from WTC 1, where steel framing has risen to the 72nd story, to Towers 2 and 3, where only basement core walls are under construction.

As heat and humidity soars, construction on Tower 3 is still not high enough to catch any breeze. Workers are sweltering amid the steel they assemble.

'This is why we went to war'

"It's hot as bulls," Mohamed says of the work.

The job is dirty and the hours long as contractors race to meet construction goals. The black rebars are oven-hot and the exposed wire ties that hold the bars together are like razors.

Ironwork is dangerous. Men dangle high above Manhattan on a soaring framework, but helmets, harnesses and roving inspectors help prevent accidents.

Mohamed says it does not compare to Iraq, where mortar rounds descended like Russian Roulette during dinner or an unseen sniper would squeeze off a round.

"Over there, you just don't control when somebody is going to attack," he says. "It's just different."

On a recent morning, as Mohamed hangs from a harness shoving a length of rebar into a towering mix of rebar over which concrete will be poured, crew boss Grogan shouts over the cacophony of machinery and worker commands.

"This is called the core," he says. "This is basically what's going to support the whole building...I mean, no way a plane's going to knock this down."

The new construction is also something more.

"A symbol," Grogan says. "This is why we went to war. They (the veterans) served their country. And now they're serving their country again rebuilding this."

A debt to the government

Mohamed, who grew up Roman Catholic, chose to serve in the military almost straight out of high school. Patriotism in response to the terror attacks was one reason. He also wanted to repay a government that had literally saved him, he says, and he wanted to flee a dysfunctional home life.

When he was 12, Mohamed says, his heroin addict father died of AIDS contracted from sharing intravenous drug needles. His mother died of cirrhosis of the liver four years later, so he went to live with extended family.

He was diagnosed as a teenager with cancer of the lymph nodes; Medicaid paid for the chemotherapy, and the disease went into remission.

"The government saved my life," Mohamed says. "So I felt like I owed them something."

He was trained as a medic in the Army and arrived in Iraq in 2004. He would eventually emblazon his right shoulder with a hand-size tattoo of his regiment, the Fighting Sixty-Ninth, a garish crest dating from the Civil War with Irish wolfhounds, red shamrock and the motto: "Gentle When Stroked, Fierce When Provoked."

His battalion would see more than 100 casualties in one year, including 19 killed, fighting in Taji and then taming the most dangerous road in Iraq — a passage known as "Route Irish" that links Baghdad to its international airport.

One night a series of buried artillery rounds blew up an armored vehicle. The blast killed six Louisiana National Guard soldiers and Mohamed's close friend, Spc. Kenneth VonRonn, 20, of Bloomingburg, N.Y.

Mohamed was among those who had to collect the remains after the explosion.

'It's, like, amazing'

After he returned home in late 2005, he was diagnosed with post-traumatic stress disorder.

Quick with his fists, he vented the anger and bitterness from war and childhood by getting into fights — on the subway, on street corners. The 5-foot-1 former medic always found some remark or a look or a gesture that begged a response.

"Ricky thinks people are always talking about him," says his wife, Jennifer, who was Mohamed's high school sweetheart. "He can't talk it out or ignore it or let it go."

He was arrested several times. Prosecutors often reduced the charges to misdemeanors because of his military service, he says.

Then came the birth of his son, Alexander Farrell Mohamed. His mom and dad sometimes refer to the 18-month-old boy as "Mr. President" — because they say anything is possible in this country.

With the help of the Helmets to Hardhats program — which funnels combat veterans into construction jobs — he became a member of the Metal Lathers, Reinforcing Ironworkers Local 46, and he started work on Tower 3 in May.

"It's, like, amazing," says Mohamed, who lives in Copiague on Long Island. "It really is, to be part of something so huge."

He says he hasn't been in a fight for a year, and his wife believes the job has something to do with it.

"He feels good going to work, and he's proud," she says.

Working at the World Trade Center for some people is "just a job," Mohamed says. "But for us (veterans), it's really, really personal. It runs deep."