A 16-year-old Arizona high school student has been charged with second-degree murder after he stabbed a fellow student to death with a pocket knife in class in front of other students.

The Phoenix Police Department said that Chris Aguilar stabbed Michael Montoya II, also 16, with a 4-inch folding pocketknife then ran out of the classroom and dropped the knife. He was later detained by a school resource officer.

Aguilar was bleeding from the hand, and he and his victim were taken to separate hospitals. Montoya died from his injuries.

Court documents say the stabbing stemmed from an altercation between the two teens a day earlier, when Montoya allegedly robbed Aguilar of a firearm, AZFamily reported. A witness told police that Aguilar and his twin brother “made statements to murder the victim after the robbery,” investigators said.

Teachers and students told investigators about the assault, which took place inside a Maryvale High School classroom just before 11 a.m. on Tuesday. Two teachers told detectives that Aguilar “was the aggressor and Montoya was not able to defend himself, the court documents said.

A classmate told detectives that Aguilar usually sat behind her in class, but on Tuesday he moved seats to sit next to Montoya. She said Aguilar punched Montoya several times before pulling out the pocketknife and stabbing him three times.

Another witness reported hearing the commotion and seeing Aguilar attack while Montoya was sitting down. Montoya tried to run away, the witness said, but he fell and Aguilar continued to attack.

It’s not clear how Aguilar brought a knife into the school since the Phoenix Union High School District uses metal detectors that should have been able to pick up a pocketknife.

>“The district’s investigation is ongoing and we continue to cooperate fully with the police investigation, a piece of which includes how the weapon got onto the Maryvale campus,” district officials said a statement on Thursday. “We do not have any further comment at this time.”

Aguilar is being held on a $500,000 bond.