MONTERREY, Mexico – Additional police guarded the U.S. Consulate in Monterrey on Monday as investigators analyzed a security video in search of assailants who shot at the building and threw a grenade that failed to explode.

Two men attacked the consulate in northern Mexico around midnight Saturday when it was closed, the consulate said in a statement. Nobody was hurt in the assault, which left the gate pockmarked. 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 there was 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.

Mexico is caught in a rising tide of drug-related killings that have been fueled by warring drug cartels and a government effort to crack down on them. Thousands have been killed so far this year.

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

Late Sunday, assailants carried out attacks in two other locations in western Mexico.

In Guadalajara, assailants threw two grenades against the state Public Safety office, shattering windows and injuring six people on the street, according to a police statement.

Authorities found the abandoned cars used by the assailants, but had not made any arrests or determined a motive.

It was the second attack on the Guadalajara office in less than six months. In June, explosives killed a policeman and wounded another. Four former soldiers were arrested in that attack.

Also late Sunday, four men in a vehicle threw four molotov cocktails at an arcade in Uruapan, a few hours outside of Guadalajara. Only one exploded, and employees and customers put the fire out. No one was injured.

In the border city of Tijuana, gunmen stormed a restaurant and shot dead two men seated at a table, as well as the restaurant's photographer. In another part of town, the bullet-riddled bodies of two men were found in an abandoned car.