NYC student killed in public school for 1st time in nearly 25 years An 18-year-old was taken into custody and is being questioned.

 -- A 15-year-old boy is dead and a 16-year-old boy is in critical but stable condition after they were both stabbed in the chest, allegedly by a fellow student in the middle of class at their New York City school this morning, police said.

The 18-year-old suspect was taken into custody and is being interviewed after the deadly incident in the Bronx building, which houses the Urban Assembly School for Wildlife Conservation and PS67.

Police said the stabbing appears to be the result of an argument involving the three students that had been going on for about two weeks.

A knife with a 3-inch blade and a 4-inch handle was recovered at the scene, police said.

The school does not have metal detectors, officials said, adding that this knife would have been picked up if there were.

"Our hearts are very heavy right now," New York City Mayor Bill de Blasio said, adding, "My heart goes out to the families who have been affected by this tragedy."

The mayor said the school community is "reeling."

Officials said there will be random checks with metal detectors tomorrow and additional school safety officials will be brought in.

No charges have been filed against the 18-year-old suspect at this time; a city official said detectives have not been able to contact the suspect’s mother because she is believed to be in Puerto Rico and has not been heard from since Hurricane Maria.

Today marked the first time in nearly 25 years that a New York City public school student was killed inside a school. The last time it happened was Feb. 24, 1993, when a 15-year-old was fatally stabbed in the hallway of Junior High School 25 in Manhattan.

ABC News' Mark Crudele contributed to this report.