MONTERREY, Mexico (AP) — Five people were trampled to death early Sunday when a gunshot fired at a cattle fair sent a panic-stricken crowd rushing for the exits in a northern Mexico town already on edge from rampant drug violence.

At least 17 other people were injured at the fair in Guadalupe, outside the industrial city of Monterrey, said Adrian de la Garza, head of the Nuevo Leon State Investigative Agency.

The dead, two women and three men, lay outside the building surrounded by crumpled beer cans and other litter dropped in the rush.

Police found a bullet casing from the shot that likely caused the 1 a.m. stampede, de la Garza said at a news conference.

At least 12 people at the scene were detained for questioning, according to an agency official who spoke on condition of anonymity because he was not authorized to talk publicly about the case. He said Guadalupe officials were investigating possible negligence by organizers of the fair, which featured live music and games.

Guadalupe Mayor Ivonne Alvarez said the rest of the event has been canceled.

A surge in shootouts has shaken the Monterrey area and other parts of northeastern Mexico, where the Gulf cartel is battling for territory with its former ally, the Zetas gang of hit men.

Five people were killed Saturday night in a gunbattle between gang members and soldiers in Camargo, a small town in Tamaulipas state, which borders Nuevo Leon, according to a brief statement from the Tamaulipas government. The statement said the five killed were civilians but did not say whether any were bystanders.

Innocent bystanders have increasingly been caught in the crossfire. Two children were killed last month when the car they were riding in drove into a gunbattle between soldiers and gang members outside the Tamaulipas city of Nuevo Laredo.

In March, two Monterrey university students were killed during a shootout between soldiers and gunmen at the gates of their campus.

In a report released late Saturday, the federal Attorney General's Office said one of the students was killed by a bullet fired by gang members. No conclusions could be determined from the bullet fragments extracted from the other student, the report said.

Sunday's stampede was the second time in a month that fears of shootings sent crowds fleeing from a fair in northeastern Mexico.

On April 2, rumors of gunfire sent thousands fleeing a street fair in the Gulf coast city of Tampico where singer Jenni Rivera was about to perform. Rivera's security team rushed her from the stage.

Nobody was seriously hurt in that incident, and it was unclear if shots were actually fired.