SAN ANTONIO — A leading researcher says at least three teenage football players died as school practices started up this summer, fatalities which followed new health warnings to coaches about the risks of heat stroke.

Autopsy results are not complete yet, and three deaths is about average for the sport in most years. But athletic trainers who urged high school coaches in June to adopt tougher safety guidelines for football practice were deeply concerned nonetheless.

Several coaches at top prep programs contacted by The Associated Press said they hadn't heard of the recommendations or followed them.

Some called the guidelines, which included no two-a-days during the first week of practice, too restrictive.

"What about all those who get to the ER and are lucky enough to get saved? We want to stop it before it gets to that point," said David Csillan, a New Jersey high school trainer who co-authored the June report by the National Athletic Trainers' Association.

As this football season gets into full swing, a former Kentucky high school coach is standing trial in a player's on-field death. David Jason Stinson is charged with reckless homicide in the death of a 15-year-old Pleasure Ridge Park High School student who collapsed last year at practice. Stinson has pleaded not guilty.

Players in Maryland and Texas, along with another Kentucky student, have died following football practices this year, said Dr. Fred Mueller, director of the National Center for Catastrophic Sports Injury Research at the University of North Carolina.

None of the deaths have been confirmed as heat-related, but each has heightened awareness about heat illness. The first was a 16-year-old in Germantown, Md., player.