MEXICO CITY—Four U.S. citizens have been kidnapped after gunmen opened fire on their vehicle in the northern Mexico border city of Matamoros, the FBI said Sunday.

The four had entered Matamoros, across from Brownsville, Texas, on Friday. They were traveling in a white minivan with North Carolina license plates.

The FBI San Antonio Division office said the vehicle came under fire shortly after it entered Mexico.

“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 return of the victims and the arrest of the culprits.

Matamoros is home to warring factions of the Gulf drug cartel.

The shootouts in Matamoros were so bad that the U.S. Consulate issued an alert about the danger Friday.

Tamaulipas state police said people had been killed and injured Friday, but did not say how many.