Monday was the fifth anniversary of the day when terrorists flew planes into the Pentagon, a Pennsylvania field and the World Trade Center. If you were in kindergarten or elementary school on Sept. 11, 2001, you might remember school letting out early, the news being on television constantly and your parents feeling very sad or angry.

Whatever you do or don't remember about Sept. 11, it's a date you will know for the rest of your life. Nearly 3,000 people were killed by the cruel actions of a few that day. As the shocking events unfolded, heroes emerged — the passengers aboard a plane that crashed in Pennsylvania who fought their hijackers; the thousands of firefighters and police who responded in New York and Virginia; and the animals that helped search for survivors and those who died.

A new book celebrates the four-legged heroes of that day. "Dog Heroes of September 11th," by Nona Kilgore Bauer, tells the stories of 78 dogs that worked for days, often with little rest, at the World Trade Center and the Pentagon. The book also explains how dogs are trained for this special assignment.

KidsPost introduces you to a few of these animals. They prove that a dog really is man's best friend.

ANNA

Anna and her handler, Sarah Atlas, arrived at the World Trade Center on the afternoon of Sept. 11. Anna, a German shepherd, was trained to find people trapped alive. When Anna first arrived at the site, she seemed desperate to find someone alive, Sarah remembers. But as the days wore on, the dog seemed to pick up on the sad mood around her. "I believe because I was tired and drained, she picked up on my emotions," Sarah said.

Because Anna was also trained as a therapy dog — helping people in hospitals and nursing homes feel better — the pats and hugs she got from firefighters and rescue workers kept her spirits up.

Sadly, Anna died in 2002. In recognition of her work, a flag was flown over the U.S. Capitol on Oct. 1 that year. A certificate in her honor noted "the bravery and courage (she) displayed while serving as a rescue dog at the World Trade Center."

RILEY

Riley, a golden retriever, is one of the most famous dogs of Sept. 11 because of a photo taken of him at the World Trade Center site a few days after the attacks. In the photo, Riley is in a basket being sent over a 60-foot-deep canyon to search the rubble of the North Tower. "Normally when we send a dog, the handler goes with him," said Riley's trainer, Chris Selfridge. "This time we decided it was more practical to just send the dog."

Like Anna, Riley helped other rescuers just by being a lovable dog. "You could see the people change from being very down to wearing little smiles ... even for just 10 seconds," Selfridge said.

STRYKER

Stryker is a German shepherd who works with the Maryland-National Capital Park Police and helped at the Pentagon for 11 days following the attacks. Alice Hanan, Stryker's handler, said he seemed to understand the importance of the job because "he lost his normal playful attitude."

Stryker's hard work didn't begin or end at the Pentagon. He has been named the park police force's K-9 Officer of the Year five times.