There, sniffing packages, garbage cans and even commuters at a downtown subway stop is Gillis - a 3-year-old black Labrador retriever named after New York Police Department Sgt. Rodney Gillis, who died when the south tower of the World Trade Center collapsed minutes after he ran inside.
Spencer is among more than 530 police officers who work with bomb-sniffing dogs named after police officers, firefighters and others killed when they responded to the 9/11 terrorist attacks. His job, like that of other officers assigned to train stations, airports and bus terminals, is to prevent another attack.
"All these people were victimized by terrorist attacks and these dogs, that's exactly what they fight against," said Spencer, 45. "These dogs are on our front lines against terrorism."
Defending those lines took on a greater sense of urgency after terrorists hijacked jets and slammed them into the World Trade Center and the Pentagon.
Authorities realized they needed bomb-sniffing dogs at all sorts of transportation centers. After toying with the idea of giving them funny names or ones tied to mythology, the Transportation Safety Administration settled on names that would ensure nobody would forget the dogs' purpose, said Scott Thomas, manager of the TSA's Canine Breeding and Development Center.
"The best thing about naming the dogs after 9/11 victims is it keeps us on task," he said, adding that some dogs have been named after military personnel killed in Afghanistan and Iraq.
The program adds an extra first letter to each dog's name to identify it as a participant.
Spencer said focus is only part of the story. He had just returned from Lackland Air Force Base in Texas, where he went through the TSA training with Ggillis last year, when he encountered a New York police officer outside Wrigley Field.
The officer and his buddies were in town to see a Cubs game and asked where they might find a good place to eat.
"He heard me say her name and said, 'That's a weird name for a dog,'" Spencer said. "When I told him, 'She's named after one of your guys,' he gets choked up."
The officer knew Rodney Gillis.
Spencer rushed to New York with some other Chicago officers after 9/11 to help out on jobs like directing traffic and searching for human remains at Ground Zero.
He said he's wanted to call the Gillis family. He wants them to know of his admiration for Gillis, who was off-duty when the planes struck but raced to Ground Zero and made it about 20 floors up the south tower before it collapsed, according to radio reports.
He wants them to know he's honored to be partnered with a dog named after such a hero and that they'd be proud of the dog. But he's never quite figured out how to put it.
When it became clear his black Labrador retriever would not wash out of the highly selective program as many do, he decided to contact Stack's son to tell him what a "privilege" it was to be teamed with Sstack at the city's airport.
But he wanted to do it in a way that wouldn't intrude like a phone call might, or with an e-mail, which is so easy to send that he worried it might be seen as a quick message that didn't mean much to him.
He wrote a letter. "As your father's job was to ensure the safety of his fellow firefighters, Sstack and I will ensure the safety of airport employees, passengers, police officers and firefighters at SFO," he wrote.
"About a week later I got a call from Michael (Stack's son, a New York firefighter) and he left a message saying, 'We just want you to know we're coming to Sstack's graduation.'"
In July, Stack's mother, widow and son left New York for Texas to attend the ceremony with the 7-year-old grandson he never got to meet.
Thomas said the response from 9/11 families has been overwhelmingly positive. The TSA doesn't call families, in large part because workers don't want to add to their hurt if the dogs named after their relatives don't get as far as Ggillis or Sstack. But families call them.
"They call and say 'My family member died in the Pentagon, would you name a puppy after him?'" Thomas said.
Officers say the dogs provide an opportunity to talk about at least one person who died on Sept. 11.
"I tell people my dog's named after someone pretty interesting," said Josh Diaz, a Chicago police officer who works with Ppearsall, a yellow lab named after New York firefighter Durrell Pearsall Jr.
"I tell them, 'Do me a favor, look this guy up, Google him.'" Gillis' family said they were honored when they learned there was a dog patrolling Chicago with a black and yellow-checkered collar signifying the rank of sergeant.
"All of the heroes we lost, they should be remembered and to be remembered in such a significant way is wonderful," said Gillis' mother, Geraldine.
His widow, Serina Gillis, agreed. And, she said her husband "would get a kick out of it. He loved dogs."
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