MATAMOROS, Tamaulipas – Military forces evacuated the administrative offices of this border city after a 911 caller claimed there were explosives in the building. The threat came one day after Mexican federal authorities in San Luis Potosi arrested the leader of the Gulf Cartel, the criminal organization that controls Matamoros.

Law enforcement sources revealed to Breitbart news that shortly after 10 a.m. Monday, someone called 911 about an explosive device left at Matamoros City Hall. State and federal authorities in addition to the military rushed to the building. It remains unclear if the mayor was on the premises at the time of the threat.

Authorities evacuated more than 500 individuals while bomb crews combed the building for an explosive device. Two hours after the initial call, authorities did not find anything and allowed all personnel back into the building.

The bomb threat comes one day after federal authorities arrested Jose Alfredo “El Contador” Cardenas Martinez in San Luis Potosi. Cardenas inherited control of the Gulf Cartel, the criminal organization that controls the region’s drug trafficking, human smuggling, prostitution, fuel theft, extortion, and kidnapping. In recent months, El Contador began fighting a turf war with a rival faction of the Gulf Cartel over control of Reynosa, immediately south of McAllen, Texas.