Updated

The FBI in Arkansas has joined a regional investigation of million-dollar robberies in the parking lots of jewelry stores, hotels and restaurants.

The latest daytime heist was Tuesday in Little Rock in which thieves made off with $500,000 in jewelry.

Agents say that robbery, matching two others last month in Pine Bluff and Nashville, Tennessee, is similar to thefts in Houston of jewelry worth at least 3.5 million dollars.

In Houston, police report at least six robberies of jewelry salesmen and couriers since April, with one May theft netting nearly 1.5 million dollars in diamonds.

Officers say the thieves used pepper spray, pistol-whipping and knives to threaten their victims.

FBI and police suspect the thefts could be part of what the bureau refers to as "South American Theft Groups" targeting diamond couriers.