Blaec Lammers was arrested this week for plotting to shoot theater-goers during a screening of Breaking Dawn Part 2, according to reports.
Lammers, whose own mother turned him in Thursday after becoming suspicious of his intentions, faces three felony charges in the alleged plot.
He was charged Friday with first-degree assault, making a terroristic threat and armed criminal action. He is jailed in Polk County on $500,000 bond.
Bolivar, Mo., Police Chief Steve Hamilton said Lammers, 23, was unstable and committed for 96 hours for a mental health examination back in 2009.
In Missouri, hospitals and law enforcement officials can request a person be held against their will for up to 96 hours if he or she appears to be a threat.
"It looks like everything was done appropriately at that time," Hamilton said.
"The average person will say 'Why was he not charged criminally?' And the reality is the law only allows so much when a person is having mental issues."
The investigation began Thursday, when Lammers' mother contacted authorities, saying she worried that her son "may have intentions of shooting people."
His apparent target? A capacity movie theater crowd during the premiere of the final film in the popular vampire series, police wrote in the probable cause statement.
Blaec Lammers' mom said her son recently had purchased weapons - two assault rifles and hundreds of bullets - that were similar to those used by James Holmes.
Holmes is the troubled gunman who opened fire inside a theater in Aurora, Colo., during The Dark Knight Rises movie in July. That attack killed 12 people.
Police wrote in the probable cause statement that Lammers was "off of his medication" and thankfully moved to arrest him before the unthinkable happened.