Twenty-three years ago, Sgt. Jason Walker had no idea he would end up with a life companion named Diesel, or that “Bring Your Dog to Work Day” would be, for him, just par for the course.

Walker was 21 when he joined the Atlanta Police Department. That was 1992, and he worked in patrol and narcotics until 2004, when he was transferred to the K9 Unit because that division — whose eight teams cover the city 24 hours a day and go on about 2,000 calls annually— needed a sergeant.

During his 12 years before joining the K9 Unit, he worked with a close-knit four-person narcotics team. But more often than not, at the end of their shifts, they went their separate ways. With Walker’s transfer to the K9 division, all that was about to change.

He still remembers the 11½-hour drive one-way to Miami to pick up Diesel, a black Belgian Malinois that was a puppy named Rudy at the time.

The vendor, a sort of middle man who travels to Europe to pick out potential K9s, had brought back a few choices. Each K9 had to have its own passport booklet, just like a person’s, since the dogs were traveling between countries. Some even had tattoos in their ears to mark their import.

For four or five hours, Walker and his road trip companion tested Rudy and another dog, Chico, in drive and temperament. Walker fell in love with Rudy, who was happy and energetic.

Chico, meanwhile, ended up biting Walker’s left hand so hard the officer had to get stitches.

On the long drive back to Atlanta, an excited Rudy, about 13 months old at the time, never once laid down or went to sleep — Walker thinks he was trying to figure out their final destination. A couple of weeks later, Walker did two important things. He took his dog to the APD vet for blood work, X-rays and a full work-up. And, with the help of his fellow officers, he decided on Rudy’s new name: Diesel.

The dog’s puppyhood was full of mischief, like the time he weaseled his way to the front of Walker’s APD K9 van and stole his McDonald’s Egg McMuffins. Or the time he snuck up there and drank Gatorades and a gallon of water. Or the time he tore a bunch of wires out the back of the van. (It didn’t impair the vehicle’s function, and Walker still doesn’t know what those wires were attached to.)

As Diesel gained experience, he gained maturity. But he didn’t lose his mischievous streak. At the United States Police Canine Association K9 trials in Guntersville, Ala., part of the certification process, Diesel broke away from his handler to dash into a nearby lake.

The sky is deep blue on the night of July 8 at the Atlanta Police Department’s K9 training facility. Crickets are chirping, and the shadows of Walker and Diesel are elongated on the pavement. They’re about to enter the field to demonstrate a narcotics detection training session.

Walker reins in Diesel on a short leash, giving him commands in German, the language the dog became accustomed to in Europe. As they walk past equestrian jumps reserved for training the Atlanta Police Department’s horses, Walker asks his partner, “Ready to go work?”

Diesel responds with two barks, and off they go — around and around a white Ford van, circling an APD vehicle, pasta dingy white car with a broken side mirror, and finally straight to a tealsedan. Diesel barks and paws the seam between the doors while Walker becomes giddy with pride. “Good boy! Good boy!” Diesel has sniffed out 15 grams of marijuana.

The bond between officer and K9 is crucial, Walker says, because they must rely on one another. Each dog, as a puppy, goes through a two-week bonding process with its officer. The dog lives with its officer, rides with its officer, and is fed and cared for exclusively by its officer. In short, it learns to look solely to its officer to meet all its needs by the time training begins toward certification as a K9 police dog.

While narcotics training usually lasts 12 weeks, it’s essentially simple. The dogs are raised adoring a specific toy, usually a Kong rubber cone — and in Diesel’s case, a red one. The officer hides the toy, and the K9 becomes a master at finding it. Next, the officer hides drugs and the toy together. The K9 becomes skilled at finding both. Then comes the curveball: the officer hides the drugs without the toy. The K9 finds the drugs due to association, then gets rewarded with its beloved toy.

Officers are able to certify some dogs in the midst of their training, while other K9s have to complete the entire process before they qualify. And certification is crucial. Without it, any evidence uncovered by a police dog would be inadmissible in court.

Diesel was trained in June 2006, and Walker says the process was easy. Diesel is a great, high-energy working dog, Walker says, and after nine years together the two act as a cohesive unit on autopilot.

In 2012, when Diesel was 7, he learned more crime-fighting skills when Atlanta’s new police chief decided that dogs originally trained to perform a single function — narcotics tracing — would be additionally trained to track humans and locate missing evidence, such as weapons.

Diesel learned two new jobs in a matter of six or seven weeks, when he earned his certification at age 50 in dog years. Apparently you can teach an old dog new tricks — ones he can use to help solve a murder.