An 18-year-old student was arrested Wednesday after he fatally stabbed a fellow classmate and critically injured another inside their Bronx classroom, investigators said.

It happened around 10:45 a.m. during a third period history class inside Urban Assembly School for Wildlife Conservation, a public junior and senior high school located at 2024 Mohegan Avenue in East Tremont.

NY1 was told that an argument broke out in the class, leading to a scuffle before the victims were stabbed.

Police said Abel Cendeño, 18, stabbed the two boys. He faces a slew of charges that include murder and assault.

Matthew McCree, 15, was killed, stabbed in the chest with a three-inch switchblade knife, according to police.

Sources said Ariane Laboy, 16, was stabbed in the side and chest with the same weapon and was taken to St. Barnabas hospital and is listed in critical condition.

15 to 20 students witnessed the attack. After leaving the classroom, Cendeño handed the bloodied knife to a school counselor he met in the hall, then went to an assistant principal's office and quietly waited for authorities to arrive, police said.

Students described tense moments huddled in closets and in classrooms while the school was locked down, wondering what was happening.

"The guidance counselor couldn't even keep herself calm,'' said eighth-grader Abbie Mincey. The counselor told them: "I've never seen so much blood in my life.''

"I heard the police sirens, and I heard kids screaming, I heard someone screaming," one student said. "I didn't think that it was serious until I found out what was really going on and I heard the police sirens."

Police are looking into the motive for the stabbing, but NY1 was told this may have been related to bullying and that Cendeño and the victims were involved in an argument that began two weeks ago. They had been tossing paper at one another shortly before the stabbing, authorities said.

"I wouldn't say they're bullies, but I guess you can say they kind of like to start a little trouble," one student said about the victims.



"I like he got provoked," the student added about Cendeño.

But others said McCree was no bully, and did not deserve to die. "He was a good kid, he was good influence," a friend said. "If it wasn't for him, I wouldn't be here today. He got me through a lot of things. I'm just speechless."

The building where the stabbing took place also houses P.S. 67, an elementary school for grades pre-K to five.

Officials said the building, which educates more than 1,000 students, did not have metal detectors.

Mayor Bill de Blasio said random metal detector scans will be conducted at the school starting Thursday. Grief counselors will also be on-site.

Angry parents, some in tears, gathered outside the school, demanding they be allowed to pick up their children. Parents said they were forced to wait for hours in fear.

"I'm very upset. No one wants to send a child to school to be in danger,'' said Rosalyn Valoy, who picked up her fourth-grade daughter about four hours after the stabbing.

Denise Jackson, the mother of a high school freshman, said she was terrified.

"She hasn't been here for a month yet. I don't know if I want to keep her here,'' she said. "I just don't know.''

"It's unacceptable to ever lose a child to violence in a school building, and we will redouble our efforts to protect every child, we will use every measure to make sure every child is safe," the mayor said.

Schools Chancellor Carmen Fariña released a statement saying, "I'm heartbroken at the loss of one of our students and this tragedy should never have taken place. We are working closely with the NYPD on the investigation and providing ongoing guidance support and counseling to the school community."

It was the first homicide inside a New York City school since 1992, when two teenagers were shot to death in the hallway of a Brooklyn high school about an hour before Mayor David Dinkins was to visit the school to talk about violence.

City officials and parents have debated for years whether the school system should be installing more metal detectors or taking them away because of the stigma of attending a school deemed unsafe enough to require a weapons check.

Deadly violence inside city school buildings is rare, though there has been violence outside, on school property. In 2014, a fight between two 14-year-old boys ended with one stabbed to death outside a Bronx school.

Giselle Estevez, the mother of 9- and 13-year-old girls, said she'd seen violence at the school before and this latest attack was the final straw. She plans to pull her daughters out of school.

"There is too much fighting, too many older boys,'' she said in Spanish. "They're crying, wondering what is going on, and the school didn't even call me."

An investigation is ongoing.