State electoral board chairman Jesus Leopoldo Lara said the killings were not related to Coahuila's local elections, in which voters were choosing all 38 mayors in the state, across the border from Texas.

There was no immediate information on a motive, but Coahuila is one of the Mexican states where drug gangs are active. Local news media said the victims were tied up, beaten and tortured, then dropped outside the school.

Local elections were also being held Sunday in the southern Gulf Coast state of Tabasco, where soldiers in the town of Cardenas took control of the local police station after municipal officers seized the office in a labor dispute. About 200 police were demanding better pay and benefits.

In Chihuahua state, located next to Coahuila, the Mexican army reported Saturday that it had captured two men allegedly involved in dozens of gang killings.

The Defense Department said the first man was captured in the border city of Ciudad Juarez on Friday after soldiers spotted him driving a pickup truck without license plates. They found he was carrying a pistol, it said.

The department said the man confessed to having participated in 55 killings this year while working as a hit man for the Linea gang, part of the Juarez drug cartel. The army quoted him as saying the victims were members of rival gangs.

The suspect also participated in shaking down local business owners for protection money and burning stores of those who refused to pay, the statement said.

In the second case, the army said troops arrested a man Friday who may have been involved in as many as 30 killings.

The army said the suspect was detained in Ciudad Juarez after he was spotted carrying a suspicious-looking bag and tried to flee. An inspection revealed the man was carrying an assault rifle, the army said.

The army said the man also confessed to working for the Linea gang and having participated in about 30 execution-style slayings.

It said the suspect was born in Los Angeles, California, but U.S. consular officials were not immediately available to confirm his nationality or hometown.

Both men were turned over to civilian prosecutors pending charges. There was no attorney of record listed for them.