A commando attacked a hotel in which state police officers were staying in Nuevo Laredo, Tamaulipas, early Friday, killing one officer and wounding two others.
State police said Northeast Cartel sicarios attacked the Santa Teresa Hotel with assault rifles and fragmentation grenades.
The aggression took place a few hours after Federal Police officers transferred 19 prisoners considered to be high-ranking members of the Northeast Cartel from municipal jails in Nuevo Laredo.
The cartel, which controls Nuevo Laredo, was formed after a split in the Zetas cartel and is led by Juan Gerardo Treviño Chávez, the brother of former Zetas leader Miguel Ángel Treviño Morales.
The assault on the hotel is the third attack on police in northeastern Mexico this week. On Wednesday, a group of Coahuila state police were ambushed on a highway near Piedras Negras, and were later fired on again as the wounded from the first attack were being taken to hospital.