First responders from across America came to New York City by the thousands within hours of the Sept 11 attacks. Then came the four-legged ones.

Riley the golden retriever, Coby and Guinness, Labrador retrievers, from Southern California. Thunder from Lakewood, Washington, and about 100 other search and rescue teams and their dogs from around the country arrived at Ground Zero to join the search for survivors. Some went to sites where the Islamist terrorist group al-Qaida crashed other hijacked planes — the Pentagon in Washington DC, and to Shanksville, Pennsylvania.

The first dogs to arrive at Ground Zero in New York City were from the New York Police Department's (NYPD) K-9 urban search and rescue team, arriving at the South Tower just 15 minutes after its collapse.

The dogs were exposed to conditions that could have killed them. They climbed atop moving piles of rubble. They sniffed deeply on the ground where toxic substances released from the two destroyed buildings were most highly concentrated. They allowed themselves to be lowered on swinging platforms to areas where human searchers couldn't go.

Though survivors were found in the rubble, the NYPD reported that none was the direct result of a dog's discovery. Several people, though, have credited Trackr, a German shepherd, with having played a role in the rescue of Genelle Guzman-McMillan.

The Port Authority worker on the job for only nine months worked on the 64th floor of the South Tower. She had walked down to the 13th floor when the building collapsed, trapping her for 27 hours under concrete, steel and dead bodies.

Trackr's handler, a Canadian policeman who drove down from Nova Scotia, was suspended from his job for leaving without permission when his department saw him on television aiding the rescue efforts.

Riley was trained to find any survivors who might have been trapped. Though he didn't find any, he located the bodies of many firefighters.

"I tried my best to tell Riley he was doing his job," his handler said. "He had no way to know that when firefighters and police officers came over to hug him, and for a split second you can see them crack a smile — that Riley was succeeding at doing an altogether different job. He provided comfort. Or maybe he did know.''

The black and yellow Labrador retrievers Coby and Guinness worked 12-hour shifts in 11 days alongside rescue workers, finding the human remains of dozens of people beneath the rubble.

Sage, a 2-year-old black and white border collie, searched through the Pentagon. She ended up finding the cadaver of the terrorist who flew American Airlines Flight 77 into the building.

"Willow and I were one of four dog teams deployed with the Pennsylvania Task Force," recalled Willow's handler, Bobbie Snyder, of Williamstown, New Jersey. "We searched the rubble in 12-hour shifts until they told us we were no longer needed."

Some of the dogs went beyond searching the rubble and became comfort dogs.

Bretagne (pronounced Brit-nee), a 2-year-old golden retriever, was one of them. She worked with her owner and handler Denise Corliss, who was a volunteer firefighter, at Ground Zero for 10 days.

Corliss, of Cypress, Texas, remembered how her canine partner interacted with the other rescuers. "Many of the rescuers I met on location would stop by to see Bretagne," she said. "While petting her, [they] would tell me their stories about the loved ones who were missing. The stories were heartbreaking."

Bretagne remained the only living 9/11 search and rescue dog until June 6, 2016, when she was laid to rest just shy of 17 years old.

As Bretagne entered the animal hospital in Cypress, Texas, firefighters and search and rescue workers from the fire department lined the sidewalk and saluted. She was carried out later, her body draped in an American flag.