In September 2001, amid the twisted pile of steel beams, concrete and ash where the World Trade Center once stood, 300 or so search dogs worked long hours and used their powerful noses to try to find survivors.

On Monday afternoon, the last of those search dogs died at age 16 with her longtime handler and best friend by her side.

Denise Corliss is pictured with her search dog Bretagne at Ground Zero in New York City in September 2001. It was their first deployment together. Courtesy of Denise Corliss

Bretagne (pronounced “Brittany”), a whip-smart golden retriever with feathery fur and a sunny smile, lived an adventure-packed life until the very end. In recent weeks, though, she began experiencing kidney failure and slowing down. When Bretagne failed to do her favorite thing for three consecutive days — eat — her owner Denise Corliss realized it was time to say goodbye.

“She was really anxious last night and she just wanted to be with me,” Corliss told TODAY on Monday. “So I laid down with her, right next to her. When she could feel me, she could settle down and go to sleep. I slept with her like that all night.”

Even after her formal retirement from search work at age 9, Bretagne still loved to work. This photo was taken at a disaster training site for dogs in Texas in July 2014. Courtesy of Denise Corliss

Denise Corliss and her husband Randy Corliss brought Bretagne to Fairfield Animal Hospital in Cypress, Texas, where the dog received a special send-off from admirers of her years of service. In addition to scouring the pile at Ground Zero in New York after the 9/11 terrorist attacks, Bretagne and Corliss deployed as a search team in response to Hurricane Katrina, Hurricane Rita, Hurricane Ivan and other disasters.

Representatives from Texas Task Force 1, the Cy-Fair Volunteer Fire Department and other agencies stood at attention and saluted Bretagne as she entered the veterinary office on Monday afternoon. They saluted Bretagne again as she departed the animal hospital with her body draped in an American flag.

Firefighters salute Bretagne and her longtime handler, Denise Corliss, as they enter the Fairfield Animal Hospital in Cypress, Texas on June 6, 2016. Bretagne was euthanized at the veterinary office. Karen Warren / Houston Chronicle via AP

“This was a very small way for us to pay tribute to a dog who truly has been a hero,” Cy-Fair Volunteer Fire Department Captain David Padovan told TODAY. “Just because she’s a K9 doesn’t make her any less part of our department than any other member.”

RELATED STORY: 9/11 Ground Zero search dog still lends a helping paw

Bretagne looks back and smiles at everyone on her way into the vet's office. Karen Warren / Houston Chronicle via AP

Bretagne was transported in a formal procession from Fairfield Animal Hospital to Texas A&M University in College Station, Texas. There, she will undergo an autopsy as part of a long-running study of 9/11 search dogs.

“(Bretagne's) partnership with Denise Corliss was magical,” said Dr. Cindy Otto, a veterinarian with the Penn Vet Working Dog Center who has spent years tracking the health of 9/11 dogs. “The two of them touched lives throughout their careers together, not only in search and rescue but even after her retirement.”

Denise Corliss and Bretagne take a break together at Ground Zero in 2001. The pair worked 12-hour shifts at the site for nearly two weeks. Courtesy of Denise Corliss

Bretagne retired from formal search work at age 9, but she never lost her love of adventure or her work ethic. Her retirement years turned out to be almost as epic as her younger years, in large part because Corliss realized Bretagne needed tailor-made physical and mental stimulation as she aged.

Bretagne and Denise Corliss take a quick nap amid the rubble of the World Trade Center in 2001. They and other volunteer search teams tried hard to find human survivors, but they only found human remains. "I really believed we could find somebody — anybody! — if we could just get to the right void space," Corliss said. "But our reality was much different." Courtesy of Denise Corliss

At age 13, Bretagne began experiencing so much stiffness and joint pain that she could no longer climb the stairs in her home. Corliss installed an above-ground pool in her backyard and helped Bretagne to swim in it for at least 10 minutes a day.

Denise Corliss installed a pool in her backyard to help her aging dog Bretagne maintain mobility. Courtesy of Denise Corliss

“It makes a huge difference,” Corliss recalled. “She started doing the stairs again. Then we started focusing on ways to keep her mentally active. ... Helping kids with their reading in school (was) great for that.”

Until very recently, Bretagne volunteered as a reading assistance dog at an elementary school near her home. She also swam regularly and took daily walks around a pond, where she loved to chase squirrels and ducks.

Bretagne loved helping first-graders practice reading out loud at an elementary school in Texas. Courtesy of Denise Corliss

Bretagne made national news at age 15 when she returned to Ground Zero with Corliss for the first time since the 2001 terrorist attacks. NBC News' Tom Brokaw interviewed Corliss at the 9/11 Memorial and also spent time with Bretagne, who was a 2014 finalist for the American Humane Association's annual Hero Dog Awards.

Bretagne generated even more headlines when she turned 16. In honor of Bretagne’s birthday in August 2015, BarkPost coordinated an epic “Sweet 16” bash in New York City that included an illuminated billboard in Times Square and the dedication of a cobblestone in her honor on the plaza of the 9/11 Memorial.

In late 2015, Bretagne became a star of a non-fiction book about senior dogs and met with former President George H.W. Bush at the George Bush Presidential Library in College Station, Texas.

“She just keeps on going and enjoying life,” Corliss told TODAY in March of this year. “She’s just such a happy dog.”

Denise and Randy Corliss' highly trained golden retrievers, pictured from left to right: Taser, Aid'N and Bretagne. Courtesy of Denise Corliss

Bretagne's life story is chronicled in the non-fiction book "My Old Dog." New World Library

Bretagne would have turned 17 on Aug. 25. Just one month ago, Denise and Randy Corliss had to say goodbye to another of their dogs: Aid’N, a retired search dog who died of cancer at age 11. The Corlisses still have Taser, an active working search dog.

“Their personalities are all so different, and I’ve tried hard to capture memories — snapshots — of their personalities,” Corliss said. “There are just so many little things that I’m really going to miss.”

Bretagne's life story is featured in the bestselling book "My Old Dog: Rescued Pets with Remarkable Second Acts" by TODAY writer Laura T. Coffey. Bretagne's chapter includes comments from NBC News' Tom Brokaw and exclusive photographs of Bretagne at the 9/11 Memorial taken by Lori Fusaro.

Here’s a look back at other 9/11 Ground Zero search dogs who lived exceptionally long lives. To read memorial tributes to almost all of the 9/11 dogs who worked in New York and at the Pentagon, click here.

Merlyn is a search dog who worked at Ground Zero after 9/11 and lived to be 16 years old. Merlyn's handler was Ann Wichmann. Penn Vet Working Dog Center

Kaiser the search dog and handler Tony Zintsmaster are pictured at the pile at Ground Zero in 2001. Kaiser lived to be 15.

Lucy also worked the pile at Ground Zero. She lived to be 15. Her handler was Lynne Engelbert. Penn Vet Working Dog Center

Ground Zero search dog Mizu lived to be 15 years old. Mizu is pictured here with handler Billy Kidd. Penn Vet Working Dog Center

Terry Trepanier, a career firefighter, is pictured in 2002 with his search dog, Woody. The pair worked at Ground Zero from Sept. 11 to 17, 2001 with Ohio Task Force 1. Woody died on Jan. 24, 2015. Courtesy of Debbie Lakins

Ground Zero search dog Polly lived to be 15. Her handler was Marti Vanada. Penn Vet Working Dog Center

Tuff, a Ground Zero search dog, lived to be 14. Tuff's handler was Tom Andert. Penn Vet Working Dog Center

Ground Zero search dog Topper lived to be 14. Topper's handler was Patricia Grant. Penn Vet Working Dog Center

Ground Zero search dog Ditto lived to be 14. Ditto's handler was Elaine Sawtell. Penn Vet Working Dog Center