According to a police probable cause statement, she said she believed her son had amassed weapons similar to those used by the gunman who attacked a theater in Aurora, Colo., during the latest Batman movie in July. That attack killed 12 people.
Investigators later determined Lammers had recently purchased two assault rifles and 400 rounds of ammunition.
Lammers' parents also tipped off authorities about the previous Walmart incident. The 2009 police report said Lammers' mother called authorities after finding a suicide note from her son and a missing knife. An officer was en route when the mother said her son's vehicle had been found in the parking lot of the Walmart store in Bolivar, a town about 130 miles southeast of Kansas City.
The parents said in 2009 that Lammers had been diagnosed with depression, and his mother said Lammers had an appointment that day with his psychologist and "he has been acting different since then."
According to a probable cause statement in the movie theater case, Lammers was questioned Thursday afternoon and told authorities he bought tickets to a Sunday "Twilight" screening in Bolivar and planned to shoot people inside.
Lammers also said he planned to "just start shooting people at random" at the Walmart store less than a mile away, so if he ran out of bullets, he could "just break the glass where the ammunition is being stored and get some more and keep shooting until police arrived," investigators wrote.
The probable cause statement said Lammers was "off of his medication," but his mother said Tuesday that to her knowledge, that wasn't true.