A man grabbed a police officer's gun and began shooting, wounding five policemen outside the statehouse of the northern Mexico state of Coahuila. Three officers were reported in serious condition.

State security spokesman Jesus Carranza said the attacker has been detained and his motives were under investigation.

Coahuila has been wracked with drug cartel violence, but there was no immediate indication that the shooting was related to drug violence.

The wounded officers from the municipal police force were outside the state government headquarters in the capital city of Saltillo as workers set up a stage for a Mother's Day celebration Friday.

Elsewhere in Coahuila, Mexican marines detained Alfonso Zamudio, an alleged accountant or financial operator for the Zetas drug cartel. Interior Department spokesman Eduardo Sanchez said Zamudio was arrested Wednesday in the Coahuila city of Monclova.

In another northern state, Sinaloa, a large band of gunmen attacked a convoy carrying the police chief of the coastal city of Ahome. The gunbattle killed three gunmen and a police officer, and a bystander was seriously wounded, state Attorney General Marco Antonio Higuera Gomez said.

The police chief, Jesus Carrasco Ruiz, had received threats before and was travelling in an armored sport utility vehicle. The attackers first threw a grenade at the convoy, then opened fire on it a few miles away.