The gate at the U.S. Consulate in Monterrey was pockmarked with bullet holes Monday, a day after assailants shot at the building and threw a grenade that failed to explode. No one was injured.

The consulate issued a statement saying that two men launched the attack around midnight Saturday, when the consulate was closed. Six spent .45-caliber casings were found at the scene.

It was not clear if the attack was related to a wave of violence by drug gangs and no evidence it was related to political terrorism, though fears persist that international terrorists might use Mexico to attack U.S. interests.

Officials were analyzing the consulate's security video, but had no suspects and had not determined a motive.

The consulate increased security and expressed "profound concern."

Offices were closed Monday because it was Columbus Day in the U.S., but two police pickups were stationed outside the building. A guard on duty said he was at the consulate when he heard the shots fired. He ran to the front, but the assailants already had fled.

The guard, who wasn't authorized to give his name, said the grenade didn't go off because the pin wasn't removed.

Alleged drug hit men threw grenades into a crowd of Independence Day revelers on Sept. 15, killing eight in the western city of Morelia. The attack heightened fears that drug cartels were expanding their violent campaign against a Mexican government crackdown.

Also late Sunday, assailants made the second grenade attack in less than six months against the state Public Safety office in the western city of Guadalajara, injuring six people on the street, according to a statement issued by the police agency.

Two grenades exploded and shattered windows of the office, a furniture store and several cars nearby. Authorities found the abandoned cars used by the assailants, but had made no arrests and had not determined a motive for the attack.

The Public Safety office was operating normally on Monday.

An attack on the office in June killed a policeman and injured another. Four former soldiers were arrested in the June attack.