It was nearly midnight, and Saul Martinez was cleaning up Tacos Los Chemas’ kitchen, a cramped space fragrant with the scent of pork roasted on the trompo.

Just as Martinez, who owns the food truck, was about to put away a salsa container, he saw two men in hooded sweatshirts approach. He didn’t give them a second thought, assuming they were hungry customers looking for a last-minute bite.

Then, Martinez later recalled, a commotion erupted on the sidewalk.

He saw one of the men jab a handgun into the back of a taquero’s head.

“Give me all your money or I’ll kill your friend,” the gunman told Martinez.

Martinez emptied his apron, handing over the day’s earnings. It was hundreds of dollars. But the robber wasn’t satisfied.

“Toda la feria,” he kept repeating in stilted Spanish — all the money.

The Los Angeles Police Department has assigned detectives from its elite Robbery Homicide Division to probe what Deputy Chief Kris Pitcher calls an “emerging crime trend.” In all, there have been more than 20 robberies, many of which could be connected, according to LAPD officials.