LITTLE ROCK—A masked gunman and a partner stole a briefcase full of jewelry Tuesday from a courier making a delivery to a high-end jewelry store, police said.

The lunchtime robbery, along a well-traveled road in Little Rock, happened quickly—with robbers blocking in the courier’s car with a Chrysler 300 sedan, said police Lt. Terry Hastings. The armed gunman walked up to the courier’s car, smashing out a front and back window as it sat outside of Kenneth Edwards Fine Jewelers, Hastings said.

“He told him to lay over in the seat and that was all he said,” Hastings said.

The robber escaped with a briefcase fill with an unknown amount of jewelry, likely including diamonds, Hastings said. He said detectives were still talking to the courier.

Tuesday’s robbery matches two others in Pine Bluff and Nashville, Tenn., where robbers stole about $1 million combined from diamond couriers in parking lots. Hastings said there was a “good possibility” the crimes bear a link and said investigators would speak with FBI agents. The June parking lot robberies, one at a hotel in Nashville, the other at a strip mall in Pine Bluff, both targeted jewelry couriers who flew into distant airports, then rented cars to travel to their sales. Both salesmen told police the attacks happened quickly, with one jeweler even losing the diamonds hidden away inside his socks. A recent bulletin by the FBI detailed “South American Theft Groups” targeting diamond couriers. The groups, crewed by illegal immigrants from countries like Colombia, Ecuador and Peru, stake out traveling salesman and jewelry shows, sometimes traveling across the country for scores.

Steve Frazier, a spokesman for the FBI’s Little Rock field office, said agents already received some details about Tuesday’s robbery. Frazier said agents were not yet investigating the theft.