A Missouri man accused of plotting to shoot up a movie theatre during the new Twilight film was charged after his mother contacted police, telling them she worried her son had purchased weapons similar to those used during the fatal Colorado cinema shooting.

Blaec Lammers, 20, is charged with first-degree assault, making a terroristic threat and armed criminal action. He was jailed on Friday Polk County on $500,000 US bond.

Blaec Lammers is charged with three felony counts, including making a terrorist threat after his mother called police saying she was concerned after her son bought two assault rifles. (Polk County Sheriff's Department/AP)

"Thankfully we had a responsible family member or we might have had a different outcome," Bolivar Police Chief Steve Hamilton told The Associated Press. He said Lammers is under a doctor's care for mental illness, and court documents said he was "off of his medication."

A phone message left by The Associated Press at Lammers' home wasn't returned Friday. No attorney is listed for him in online court records.

His mother contacted authorities Thursday, saying she worried that with this weekend's opening of the final film in the popular Vampire movie series, her son "may have intentions of shooting people at the movie," police wrote in the probable cause statement.

She said she thought the weapons — two assault rifles and hundreds of bullets — resembled those used by a gunman who opened fire inside a cinema in Aurora, Colo., during the latest Batman movie in July. That attack killed 12 people.

Lammers was questioned Thursday afternoon and told authorities he bought tickets to a Sunday Breaking Dawn: Part 2 screening in Bolivar, in the southwest corner of Missouri, and planned to shoot people inside the cinema. The town of roughly 10,000 people is about 60 kilometres southeast of Kansas City.

According to the probable cause statement, Lammers also planned to "just start shooting people at random" at a Walmart store less than a mile away. He said he'd purchased two assault rifles and 400 rounds of ammunition, and if he ran out of bullets, he would "just break the glass where the ammunition is being stored and get some more and keep shooting until police arrived," investigators wrote.

Lammers stated he wanted to stab a Walmart employee to death and followed an employee around a Walmart store before officers got involved in 2009, according to police.

When asked about recent shootings in the news, Lammers told police "he had a lot in common with the people that have been involved in those shootings," the probable cause statement said. Investigators also wrote that Lammers said he "was quiet, kind of a loner, had recently purchased firearms and didn't tell anybody about it, and had homicidal thoughts."

Obtained guns legally

Police said Lammers bought one firearm Monday and another Tuesday. He then went to the Missouri town of Aldrich to practice shooting because he "had never shot a gun before and wanted to make sure he knew how they shot and how they functioned," the probable cause statement said.

Hamilton said it appeared that Lammers obtained the firearms legally but that police were continuing to investigate "to determine how in fact he was able to obtain a permit."

Ashley Miller, who lives in a nearby town, said she has known Lammers for about a year and described him as "one of the sweetest guys I had ever met" but "very emotional," noting he would periodically stop talking to her.

She said he told her that he had bounced between relatives growing up. As an adult, he bounced between girlfriends, she said.

"He was never actually happy," she told the AP in a phone interview. "I think he had depression or something."

Polk County prosecutor Ken Ashlock said Lammers' first court appearance likely will happen Wednesday. He said his office would file a motion asking for a mental exam of Lammers. He said he wasn't aware of any charges in the 2009 Walmart incident.

Like the police chief, the prosecutor credited Lammers' mother for contacting police.

"It was a good thing they found what they found and took care of it," Ashlock said. "Everything was there as far as the weapons. He did have the weapons; he did have the ammunition ... Those things were all there, and then he made the statements to the officer about what his plans were."