Sheriff Jim McDonnell said police had arrested the 17-year-old student, who made the threat during an argument with a teacher on Friday, two days after a teenager killed 17 people in a gun rampage at a school in Florida.

They found two AR-15s -- the semi-automatic rifle used in the Florida shooting -- when they searched the student's family home in South Whittier on the outskirts of Los Angeles.

Police also found two handguns and 90 high-capacity ammunition magazines, the sheriff said.

The youth had allegedly threatened that "he was going to shoot up the school some time in the next three weeks," McDonnell said.

The threat was overheard by a school security guard who alerted the authorities.

"Deputies learned that the 17-year-old had an extensive discipline history at the school," McDonnell said. "They also learned that a Smith & Wesson semi-automatic weapon was registered to his home address."

The student's older brother, a 28-year-old army veteran, said the guns belonged to him, although only one of the AR-15s was registered under his name.

The student was arrested for making criminal threats while his brother was also detained on unlicensed weapons charges.