MEXICO CITY, June 3 -- At least six people were killed and 21 others were injured on Saturday in an attack on a bar in the city of Cuauhtemoc in Mexico's northern state of Chihuahua.

The attack took place on Saturday morning when an unknown man entered the night club named Chicho's, and opened fire on the crowd, according to the prosecutor's office. He apparently used an AK-47 assault rifle.

Paramedics for the Red Cross rushed to offer first aid to the injured before transferring them to a nearby hospital.

Municipal and state police as well as officials from the Chihuahua government have opened an investigation, and were gathering evidence about the alleged gunman.

Until now, the identity of the victims has not been revealed.

Chihuahua has been the scene of numerous scenes of violence, with Ciudad Juarez, one of Mexico's border cities, having been the epicenter of clashes between rival drug gangs.

The incident took place a day after fighting left 11 dead in the states of Sinaloa and Tamaulipas.