The minute the witness heard gunshots, a "code red" was called, the report said.
Cruz told police he was that gunman.
Related: Who is Nikolas Cruz, accused gunman in Florida high school attack?
As students fled, Cruz said, he thought up a plan to discard the AR-15 rifle and the extra magazines he had in his backpack so he could slip out with the other students. Police found the items where Cruz said they would be.
Cruz remained in jail Thursday. A judge denied bond for the former high school student. He has been placed on a suicide watch.