A 13-year-old New Jersey football player who collapsed during practice Monday and later died had an undetected heart condition, WNBC is reporting.

The preliminary results of an autopsy on Sean Fisher revealed the heart condition, according to the report.

Fisher's death was the first of two youth football-related deaths this week in New Jersey.

Douglas Morales, 17, died Tuesday night at Hackensack Medical Center of a subdural hematoma, or excessive bleeding in the brain. Morales was running a drill last Friday when he was tackled, his helmet striking the defender's shoulder pad.

Mayor Gerald Calabrese told The Record of Bergen County Morales wasn't hit real hard, but evidently the impact severed an artery that goes to the brain.

The 17-year-old Cliffside Park High School athlete was rushed to the hospital from practice on Friday, Cliffside Park Police Chief Donald Keane told FOXNews.com Thursday.

"We received a call at the police station at 11:50 on Friday," Keane said. "The caller said there was an unconscious football player on the field. He (Morales) was unconscious when we got there and we got him into the ambulance right away."

Morales' death came only a day after Fisher, of Waldwick, died after running drills at football practice on his 13th birthday.

Fisher's mother told The Record she was home making cupcakes for her son's birthday while the teen was at practice.

Dr. Merle Myerson, a cardiologist and director of St. Luke's Roosevelt Cardiovascular Disease Prevention Program in New York City, said if an athlete suddenly dies, it is generally from a cardiac cause.

"It can be from a fatal irregular heart rhythm, an abnormality of the heart muscle, a problem with the structure of the heart or how the heart is working, or how the electrical condition of the heart is working," Myerson told FOXNews.com. "The thing is, it's usually unusual, so when it does happen, it's tragic."

Myerson said more preventative screening is needed for student athletes.

"In Europe, they do it more, the doctors will give the athletes an electrocardiogram, and ask for the family's history, along with a physical exam," she said. "In this country, the whole issue is that sports are being cut, so forget health screenings. They probably just listen to your heart with a stethoscope, but shouldn't we be doing more?"

A funeral Mass will be celebrated Friday for Sean.

The boy was due to enter eighth grade next week.

"These were hometown kids — part of an extended family," Keane said. "The town is in mourning right now."

FOXNews.com health editor Jessica Doyle and the Associated Press contributed to this report.