Gunmen kidnapped four US citizens who crossed into Mexico from Texas last week to buy medicine and got caught in a shootout that killed at least one Mexican citizen, officials said on Monday.

The four were in a white minivan with North Carolina license plates. They came under fire on Friday shortly after entering the city of Matamoros from Brownsville, the southernmost tip of Texas near the Gulf coast, the FBI San Antonio division office said in a statement on Sunday.

“All four Americans were placed in a vehicle and taken from the scene by armed men,” the office said. The FBI is offering a $50,000 reward for the victims’ return and the arrest of the culprits.