NEW YORK (Reuters) - The faces of people trapped behind windows high up in the World Trade Center just before it collapsed five years ago still haunt Michael Puzziferri but the image has only stiffened his resolve as a New York firefighter.

Puzziferri was at his sister's house when he heard two planes had slammed into the twin towers on September 11, 2001 and went straight in to work. Within hours, he was leading 105 firefighters at the site where he was to spend six months.

Puzziferri, 52, commander of the Fire Department of New York's Battalion 27 in the Bronx, said in an interview that nothing in more than 20 years with the fire department prepared him for that day's events.

"It was beyond imagination or comprehension," Puzziferri said at his small fire station where engines bear the names of some of the lost firefighters.

At first the firefighters hoped they would find survivors but Puzziferri spoke of the horror of digging into rubble just to find body parts.

After the disaster, firefighters had to find ways to deal with their nightmares as well as the loss of 343 colleagues, who were among the 2,759 people who died.

The image of people staring desperately from the windows in the towers, shown repeatedly on television and in newspapers in the days after the disaster, has remained with Puzziferri.

"They appeared not to be injured but they were all trapped and they all died in the collapse or some actually jumped," said Puzziferri, a sturdy man with close cut hair and piercing eyes.

"That is one of those things that makes you want to help change the world. They were frozen and captured. This is something that bothers me and pushes me all the time.

"To me they are my responsibility ... and it's my job to make sure that does not happen."

The horror that Puzziferri and his men faced that day and worked with for months seem a lifetime away as he stands in his well-ordered fire station in a residential area of the Bronx with neighbors knocking on the door to borrow a bicycle pump.

TIGHTENED BONDS

September 11 triggered many changes in New York's fire department. Consulting firm McKinsey and Co. was brought in to review the department and suggest ways to prepare for other potential attacks and reshape the heirarchy to give firefighters more say in how the department works.

Puzziferri has been involved with this, offering ideas on coping with possible emergencies like an attack on a tunnel into Manhattan and the best ways to evacuate high buildings.

But the department, known as a macho, fearless force, also had to admit firefighters too sometimes need help to come to terms with what they face in the line of duty.

At first few of the FDNY's 11,500 uniformed officers were willing to admit a need for therapy after September 11, but Puzziferri said he knew this was important and led by example.

"We realized that the scope of this thing is just too big and beyond us," he said. "So I did it because I wanted to come back and say, 'Look, I am this battalion's commander and ... this is not a bad thing to do."'

He had insomnia for years but said he can now fall asleep without difficulty. He no longer shies away from social events as he did in the months after 9/11 to avoid the inevitable barrage of questions.

Following September 11, the number of firefighters retiring from the department spiked. In 2002, a total of 1,221 retired compared to the usual annual average of about 500.

But the number of people who wanted to join the department was as high as ever, with fire-fighting jobs the most sought after civil service posts in the city, a department spokesman said, indicating the extraordinary danger and deaths of 2001 were no deterrent.

Usually the fire department loses a couple of firefighters a year.

Puzziferri said September 11 had tightened bonds between New York firefighters and intensified their drive to protect people. "All of us can distance ourselves from it but, I have to be honest with you, it takes one second to go right back," he said.