A 14-year-old football player collapsed during practice at Mount Saint Michael Academy Tuesday and later died, police say.

Dominick Bess was taking part in cardio drills around 8:45 a.m., when emergency responders say he went into cardiac arrest. After he fell, police say a coach performed CPR until paramedics arrived to rush him to Montefiore Medical Center, where he was pronounced dead. The school is near the Westchester County border.

A source at the school says the students were exercising outside in shorts and T-shirts without pads – training that officials say is not rigorous. Authorities say they were not immediately sure whether the boy had a pre-existing condition. It was his first day of practice.

Relatives say a teammate told them Dominick seemed tired before the incident. His uncle says the family doesn't know of any pre-existing conditions either and isn't sure if the death was heat-related or if the teen was sick or pushed too hard in practice.

The teen was an incoming freshman on a partial scholarship. He would have begun classes at the private Catholic school this upcoming semester. The school, in the Wakefield section of the Bronx, has more than 800 students who come from the borough and Westchester.

The school released a statement offering "heartfelt thoughts and prayers" to Dominick's family, friends and teammates.

The medical examiner will determine an official cause of death.