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 (euro0.9 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 (euro0.65 million) in jewelry and gems at a gas station.

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