A 16-year-old high school student from Arizona has been charged with second-degree murder after fatally stabbing a fellow student with a pocket knife during class, in front of other students.

The Phoenix Police Department indicated that Chris Aguilar stabbed Michael Montoya II, who was also 16, using a 4-inch folding pocketknife. After the incident, Aguilar fled the classroom, leaving the knife behind, but was subsequently 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.

According to court documents, the stabbing was a result of a previous altercation between the two teens. The day before the incident, Montoya allegedly robbed Aguilar of a firearm, as reported by AZFamily. A witness informed police that Aguilar and his twin brother had “made statements to murder the victim after the robbery,” according to investigators.

Teachers and students provided information about the assault, which occurred inside a classroom at Maryvale High School shortly before 11 a.m. on Tuesday. Two teachers told detectives that Aguilar “was the aggressor and Montoya was not able to defend himself,” as stated in the court documents.

A classmate informed detectives that Aguilar typically sat behind her in class, but on Tuesday, he moved next to Montoya. She recounted that Aguilar punched Montoya several times before pulling out the pocketknife and stabbing him thrice.

Another witness reported hearing the commotion and witnessing Aguilar attack while Montoya was seated. Montoya attempted to escape, according to the witness, but he fell, and Aguilar continued the assault.

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.