HOUSTON – Brazen jewel thieves are robbing traveling dealers around Houston in a spree of heists that has jewelers on edge.

Police say the fourth holdup in three weeks occurred Thursday when three masked men walked into a restaurant, pistol-whipped a dealer in a corner booth and made off with $1.4 million in diamonds.

Since April 28, three other dealers have been robbed while in sitting in cars or while stopped for fuel. On Wednesday, one dealer was robbed of $1 million in jewelry and gems at a gas station.

Authorities believe the robbers are part of a South American ring that's operated in United States for at least 20 years.

FBI agents say they're doing everything they can to track down the robbers, but admit the task is daunting.