The man was identified as 24-year-old Kyle Bangayan of Pomona. He was taken into custody at his parents' Los Angeles apartment in the Little Armenia area on Sunday at about 11 a.m.
The LAPD and the FBI responded to threats that Bangayan allegedly posted on Facebook using an alias. Police say he threatened to carry out shootings at multiple elementary schools in the Los Angeles Unified School District while referencing the massacre in Connecticut.
"He was clear in his statements, and his abilities matched those statements," said LAPD Chief Charlie Beck during a news conference.
At the apartment, authorities found nine firearms, including rifles, handguns and a shotgun, along with ammunition. It was unclear whether the weapons were purchased legally or who they were registered to.
Police say the threats went beyond just venting on a social networking site. He remains in jail on $500,000 bail.