Federal officials on Friday began to investigate a medical helicopter crash in central Oklahoma that killed the pilot and a nurse and left a paramedic seriously injured, an official said.

Kingfisher Mayor Jack Stuteville said he arrived at the scene not long after the crash Thursday evening, after a farmer told him he saw the A-Star 350 helicopter tilt and spin out of control then smash into a field near the town some 50 miles northwest of Oklahoma City.

"By the time I got there it was already burned to pieces," Stuteville said. "The bodies were charred beyond recognition. It was bad."

Michael Eccard, a paramedic and the lone survivor, was about 50 yards from the crash site, he said.

"I still don't know how he did it. He must have bailed out just before it hit the ground ... there was no way he could have crawled that far," Stuteville said.

Pilot Al Harrison, 56, of Edmond, and nurse Ryan Duke, 35, of Oklahoma City, died in the crash, according to a Friday statement by EagleMed LLC, the Wichita, Kan.-based company that operates the A-Star 350 helicopter.

Allen Poston, a spokesman at the University of Oklahoma Medical Center in Oklahoma City, said Eccard, 34, of Edmond, was in serious condition Friday morning.

Stuteville said Eccard was conscious and talking when an ambulance crew arrived and that he showed no obvious signs of burn injuries.

The National Transportation Safety Board was to lead the crash investigation, helped by the Federal Aviation Administration, according to FAA spokesman Lynn Lunsford.

Stuteville said FAA investigators were at the scene.

"They're not letting anybody into the crash site, they're keeping everyone about a half mile away. There are several people on site, but they're controlling who gets in and out," he said.

NTSB investigator Jason Aguilera was not immediately available for comment.

The helicopter had been traveling from Integris Baptist Medical Center in Oklahoma City to a hospital about 90 miles away in Okeene when it crashed into the field near Kingfisher shortly after 7:30 p.m. Thursday.

The Oklahoma Climatological Survey's Mesonet site near Kingfisher shows wind gusts of between 30 and 35 mph around the time of the crash.

A spokesman for the helicopter's manufacturer, Eurocopter, with headquarters in France and American offices in Grand Prairie, Texas, did not return phone calls seeking comment.