No patients were on board the helicopter operated by Hospital Wing when it crashed in a field about 55 miles northeast of Memphis around 6 a.m., authorities said.
Haywood County Sheriff Melvin Bond said nearby factory workers reported seeing a large burst of lightning, followed by an orange glow in the area of the crash.
He said the helicopter crew was communicating with its base when radio contact was lost. The pilot had given no indication of a problem, he said.
"It was totally burnt," Bond said of the wreckage. Fire-blackened debris could be seen spread across part of the field and one rotor blade stuck straight up from the ground.
Hospital Wing, a nonprofit air medical transport service based in Memphis, said in a news release the victims were pilot Doug Phillips, 58, of Bartlett, Tenn.; nurse Misty Brogdon, 36, of Jackson; and nurse Cindy Parker, 48, of Dyersburg.
Authorities said the helicopter had flown a patient from Parsons to Jackson-Madison County General Hospital and was returning to its base in Brownsville when it went down a few miles from its destination.
"The pilot was not in contact with air traffic controllers at the time of the crash and there had been no indication of problems," said Lynn Lunsford, a spokesman with the Federal Aviation Administration in Fort Worth, Texas. Lunsford said the FAA and the National Transportation Safety Board were investigating.
"They (investigators) will look at everything from the aircraft to the weather," Lunsford said. "As the NTSB says, 'man, machine and environment."'
Rich Okulski, a supervisor in the Memphis office of the National Weather Service, said there were thunderstorms in the area at the time and weather could have played a role in the crash.
Okulski said the agency doesn't have an observer in Brownsville. But at the time of the crash, a thunderstorm was in progress at McKellar-Sipes Regional Airport in Jackson, about 25 miles east of Brownsville, and a line of thunderstorms had cleared Memphis, about 55 miles southwest.
Julie Heavrin, a spokeswoman for Air Evac Lifeteam, said from company headquarters in West Plains, Mo., that the weather at the time was considered too dangerous for their helicopters to fly.
She could not say whether the call was about the same patient who was airlifted by Hospital Wing, but said the request was for an air transfer from Parsons to Jackson at 4:02 a.m.
Keith Holloway, a spokesman for the NTSB, said a team was leaving Washington at midday to examine the crash site. He said the team will be on site for three to five days and a preliminary report would be released about 10 days later.
Hospital Wing has branches in Oxford, Miss., and Brownsville. It operates five helicopters. Jamie Carter, a company board member, said the crashed helicopter was a Eurocraft Astar model and one of the newest in the fleet.
He said it was the first company accident since it began operating in 1986.
"We are suspending operations with the service until we can get our arms around what happened," Carter said.
Improving the safety of emergency medical services flights has been on the NTSB's "most wanted improvements" list since 2008, a year when the industry suffered a record number of fatalities.
There were 41 people killed in 11 EMS helicopter accidents between December 2007 and February 2010, according to an NTSB report.
It said the pressure that crews face to respond quickly during difficult flight conditions, like darkness or bad weather, has led to increased fatal accidents.
Last fall, the NTSB urged the government to impose stricter controls on emergency helicopter operators, including requiring the use of autopilots, night-vision systems and flight data recorders.