It was Sunday morning when Minneapolis mounted police officer Aaron Morrison’s phone buzzed with a text that filled him with dread.

Diego, his frequent riding partner and one of the longest-serving horses in the mounted patrol unit, had suffered a serious leg injury. The prognosis for recovery was bleak, a veterinarian would later tell him.

Morrison drove out to the ranch where the department’s dozen horses are stabled so that he could be with Diego when the vet put him down, cradling the horse’s head in his lap one last time. “I just rested his head on my lap and comforted him until he passed away,” he said.

“It is pretty devastating,” Morrison said in an interview this week. “There’s a lot of officers who are really sad today because he’s gone.”

Since joining the department in 2009, the 20-year-old buckskin gelding had a reputation as a “kind, gentle” horse, who had earned the trust of his handlers for his calm demeanor in even the most raucous bar-closing crowds, Morrison said.

The relationship between horse and rider is uncommon, said Morrison, likening it to the bond shared by K-9 officers and their dogs. And so news of Diego’s death hit hard with the 29-person mounted unit.

“He was just probably one of our greatest horses,” said Morrison, one of three full-time officers in the unit. “When you get on a horse, it’s amazing, the people, they will come out and talk to you.”

The life of a police horse is one of hours spent entertaining children at parades, block parties and other events — punctuated by moments of intensity, as a recent bystander video showing a group of mounted patrolmen galloping toward the scene of a downtown shooting proved.

The unit announced Diego’s death on Facebook, where dozens of people left their condolences for “Officer Diego,” including other Midwestern mounted units.

“I’m so very sorry for your loss. I pray he is in heaven running and frolicking in a lush green meadow,” said one. ­

Diego was buried Sunday in the 80-acre pasture near Maple Plain where he and the other police horses trained and grazed. The unit wants to put a bronze plaque on the spot and eventually hold an honor guard ceremony befitting any fallen officer.

Diego was about two or three years from the age when most police horses retire. “We don’t want them to be worked anymore,” Morrison said. “We just want them to have a nice, quiet life, grow old and just be a horse.”