An Iowa training drill involving a mock school shooting by a teen venting anger over illegal immigration was canceled Friday after authorities said a real shooting was threatened at the high school where the drill was to take place.

Pottawattamie County Sheriff Jeff Danker said his office was notified about the threat that came to the school Friday morning from an out-of-state phone caller who threatened a shooting at Treynor High School, the site of the four-hour drill scheduled Saturday for police, firefighters and other first responders.

The caller stated something along the lines of, "'Your school shooting drill may be a reality today,"' Danker said. He said three deputies went to the school Friday morning as a precaution. It was not immediately clear when or if the exercise would be rescheduled. County emergency management officials did not immediately return messages Friday.

The drill has gained attention amid concerns raised by groups opposed to illegal immigration that say the fictitious scenario had a political agenda in featuring a teen with ties to a white supremacist group and gun enthusiasts who was angry about immigration issues.

Iowa Minutemen state director Robert Ussery said his group opposed the drill, which he said painted "people who believe in enforcing our laws as criminals," but members wouldn't have made threats against the school. "We're not geared toward violence, but toward education and trying to change the laws," Ussery said.

Exercise director Doug Reed had said Thursday that the county incorporated the immigration issue into the scenario to get Department of Homeland Security funds to cover the training. To qualify, Reed said, the exercise needed to be about terrorism -- which the federal government defines as the use of violence to intimidate or coerce a government or population as a means to further a political or social objective.

The statute that established the State Homeland Security Program says the federal department provides money to states to disburse to local agencies that meet the criteria: to help prevent, prepare for, protect against and respond to acts of terrorism.

Homeland Security spokesman Matt Chandler said the federal department gives money to states to hand out locally. Specific scenarios aren't dictated by the federal government but are developed on the state or local level, he said.

Stefanie Bond, a spokeswoman for the Iowa Department of Homeland Security, said the state office provides funding but has no involvement in the planning or approval process.

"The county puts together their own scenario that must meet federal criteria," she said.

Details of the Iowa scenario were not supposed to be made public before the exercise so as not to influence participants, but the information spread Wednesday and Thursday through social media websites.

The backdrop for the drill, according to the county's 41-page plan, is an influx of minorities into the community that has led to economic instability and sparked racial tensions. In the scenario, handwritten notes have been found at the school threatening violence against minority students: "If you don't get rid of them, we will."

One of the fictional shooting suspects is described as the son of a white supremacist whose family members are gun enthusiasts. The teen has protested with anti-immigration demonstrators and is known to have a quick temper.

The mock shooting unfolds as two teens approach a group of minority students in the school cafeteria and shout racial slurs. One makes comments to the effect "that he is tired of them moving in and stealing jobs and money from Americans," according to the exercise plan. One of the attackers shoots, then the other yells "the revolution begins."

Treynor school superintendent Kevin Elwood said he received about 100 emails opposing the drill over the past day.