Coahuila state Attorney General Homero Ramos said revenge "could be a hypothesis" for the slaying of 25-year-old Jose Eduardo Moreira, whose body was discovered Wednesday inside his pickup truck on a rural road on the outskirts of Ciudad Acuna, a town across the border from Del Rio, Texas. He had been shot twice in the head.
The shooters "knew who he was," Ramos said.
Ramos told Radio Formula on Friday that hours before Moreira was slain, a confrontation in the border city of Piedras Negras between state officers and suspected cartel members left five suspects dead, including the nephew of Zetas cartel leader Miguel Angel Trevino Morales.
He said police are following several lines of investigation.
The victim's father, Humberto Moreira, was Coahuila's previous governor and also served as national head of the Institutional Revolutionary Party, or PRI, in the run-up to July's presidential election. He was pushed out of the party's leadership last year amid accusations of mismanaging Coahuila's finances.
Ramos' office said in a statement that seven local police officers are being questioned in connection with the slaying.
Ciudad Acuna Mayor Alberto Aguirre said he made available to prosecutors all 48 city police officers working when the killing occurred. He said at least three were detained for further questioning.
Aguirre said he worked closely with the slain Moreira, who coordinated development programs in Ciudad Acuna for the state government led by his uncle Ruben Moreira.
"He was very active, and hard working," Aguirre said.
Drug cartel assassins often leave messages along with their victims' bodies, but Aguirre said his officers didn't find anything.
The federal government dispatched troops, federal police and criminal investigators to help in the investigation of the killing, which hit one of Coahuila's most powerful political families.
Coahuila, along with the northeast Mexican states of Nuevo Leon and Tamaulipas, has seen a rise of violence as the Zetas drug gang expands its control of drug smuggling routes on the border with Texas.
The Zetas, former special forces soldiers recruited to be the assassins of the Gulf drug cartel, expanded their presence from their home base of Tamaulipas into neighboring states in the last five years as they gained strength and began asserting their independence. They split from the Gulf cartel in 2010.
Authorities say Coahuila has seen a wave of violence tied to the Zetas cartel's battles with the Sinaloa cartel, allies of the now weakened Gulf cartel.