Police were initially called Monday after the classmate told his parents that he had heard the teen's remark, Phoenix police said. But the witness said he did not see any weapons that would allow the suspect to act on the comment, police said.
The following day, the witness reported the boy to Pinnacle High School officials after noticing that he had a plastic bag with a white, powdery substance with him, police said.
The teen was found with several pounds of potassium nitrate -- which can create an explosive substance when mixed with other chemicals -- with him at school, Phoenix Police spokesman Sgt. Tommy Thompson said at a news conference on Tuesday. In a statement, police said they didn't believe the school or anyone connected to the school was in danger.
The teen was taken to the police headquarters and interviewed by detectives.
"He's looking at some pretty serious allegations against him at this time," Thompson said.
Thompson added that authorities are still investigating the boy's motive and are combing through his social media accounts and looking for other potential threats or comments.
Investigators don't believe the statement was aimed at specific church or Mosque.