While there were hundreds of human heroes in the days following the September 11 attacks, there were four-legged heroes too -- the dogs who searched for survivors and bodies in the rubble of buildings and planes.

A decade later, the 9/11 dogs still alive are long retired, but there's a new generation coming up behind them.

Red, a 12 year-old Labrador who searched the rubble of the Pentagon with her handler, is among those retired as an active search dog. Her legs are not as spry as they once were but in her temperament Red still appears to have that same devotion to the search.

Not long after American Airlines flight 77 crashed into the Pentagon on September 11, Red was at work. She was 18 months old and only recently certified as a rescue dog, a rookie among more veteran canines.

Red searched the debris pile with an energy that surprised even her handler, Heather Roche.

"I never thought she would be a successful search dog and actually at six months old I found a pet home for her and had found another dog. I thought ... her personality is not what is needed for a working dog," Roche told Reuters TV.

"And then, no matter what I asked her to do -- whether it was climbing up things, going somewhere (as) I stayed far away, ladders, you name it -- she did it every single time and she did it perfectly," Roche said.

For weeks, Red navigated the hazards of the rubble piles amid the clatter and chaos following 9/11. After Red discovered dozens of bodies, Roche was sure that, among the rescue dogs working the pile, hers was one of the greats, though the dogs generally were impressive.

"They worked so hard and it was so hot and we were on a daytime shift, so it melted all of us. Just the work basically -- 12 hour shifts out in the sun."

By the time we were done every day, they slept hard ... but they were willing the next morning. They were rejuvenated and pulling on the leash to go back to work," Roche said.

Ten years on, Roche, who keeps Red with her in Annapolis, Maryland, is still deep in the tight community of canine search and rescue. She and other handlers train their dogs in a simulated disaster environment such as one in suburban Washington with an obstacle course of a fabricated rubble site made of concrete blocks and wooden pallets.

Roche says the experience of 9/11 has demonstrated the importance of training dogs in these hazardous environments.

In her retirement, Red still tags along on some search missions. She wants to work, even if her body has lost a step or two in these past 10 years.

And, just as the human first responders continue to suffer ailments attributed to their work at the World Trade Center and the Pentagon, the handlers of estimated 300 canine search and rescue teams know the dogs who worked in the days and weeks following September 11, 2001 also gave up a part of themselves.

"With her, you know she's earned the right to do anything she wants," Roche said.