"It was left in detention so he was already in trouble for some other behavior issues in the school," said Huntington Beach Police Department Public Information Officer Mitch O'Brien, who declined to discuss the contents of the note.
Several witnesses saw Casey, who O'Brien said turned 18 on Dec. 9, place the note on a desk while he was in detention, said O'Brien, who wasn't aware of any motive for the threat.
Huntington Beach High Principal Rocky Murray said assistant principal Kevin Seidel interviewed Casey. Seidel declined to comment, citing the ongoing police investigation.
Investigators searched the school and were unable to find any explosives. They arrested Casey at the school. Huntington Beach High administration followed all safety protocol by pulling Casey out of detention, interviewing him and calling the police, O'Brien said.
"The police got here immediately and determined an evacuation wasn't going to be necessary," Murray said. "He admitted to planting the note and to it being fake."
Officers searched his home and didn't find any bomb-making materials, O'Brien said.
"It's not like the kid had a big bomb factory in his home," O'Brien said. "He didn't appear to have the knowledge or materials to make a bomb."
Classes weren't interrupted during the threat, interviews and arrest, Murray said.
"We notified our teachers at the end of the day," he said. "Communication is critical for us and we need to make sure we get the right information out there."
Prosecutors are asking Casey be held on $250,000 bail. Murray declined to comment on whether the school will suspend or expel Casey or if he's had trouble at the school in the past, citing student rights and privileges.
Casey did not mention Connecticut in his note or in interviews with the school or police, according to O'Brien and Murray.
"It wasn't relevant whatsoever," Murray said. "It was a very tough day in general. …We are hoping for a smooth holiday."
Jeremiah Dobruck contributed to this story.