The joint investigation has been turned over to Los Angeles police, and there won't be any federal charges, FBI spokeswoman Laura Eimiller said.
When police and FBI agents arrived at the east Hollywood home, Bangayan's father gave permission for a search, authorities said.
Nine weapons and ammunition were confiscated, including rifles, a shotgun and handguns, authorities said.
It's not yet known if the weapons belonged to Bangayan or his father. No weapons were found at Bangayan's residence in Pomona.
The father's telephone number is unlisted, and there is no listing for Kyle Bangayan in Pomona.
The home is in a working-class neighborhood about eight miles northwest of downtown Los Angeles. Pomona is about 30 miles to the east.