The main rotor separated from the aircraft and was found north of the main crash site, and the tail was found to the southwest, National Transportation Safety Board investigator Jennifer Rodi said. The cause of Tuesday's crash hasn't been determined.

Investigators will try to reconstruct what's left of the aircraft on Thursday and Friday at the Clinton Airport, Rodi said. The NTSB also has obtained radar data that will be examined and may reveal more about the aircraft's final moments.

The Air Evac Lifeteam helicopter, a Bell 206 built in 1978, went down near around 4 a.m. Tuesday in the hills of Van Buren County, according to the company and the Federal Aviation Administration. The crash killed pilot Kenneth Robertson, flight nurse Kenneth Meyer Jr. and flight paramedic Gayla Gregory, the company said. There was no patient aboard the aircraft.

Nearby residents reported hearing an explosion and a fluttering or flapping sound, the engine revving and then metal crunching when the helicopter hit the ground, Rodi said.

Seth Myers, president and CEO of Air Evac Lifeteam, said company representatives met with families of the victims on Wednesday, and he plans to visit with them on Thursday. He said the accident has strongly affected people throughout the company who knew the victims.

The accident marked the fourth fatal accident by medical helicopters and planes this summer in the U.S., and boosted to 21 the number of people who have been killed in such crashes this year.

Rodi said it was too early to draw any conclusions about what caused Tuesday's wreck, but the investigative team was working to find out why the aircraft broke apart. She said investigators flew around the crash site, trying to determine if the helicopter clipped any treetops. She said but she hadn't had a chance to hear what they found.

The Air Evac fleet was fully equipped this summer with night-vision gear, and the company uses a risk-assessment procedure for each mission, Myers said. The company uses flight simulators for training that are scenario-based, and pilots come to the company's headquarters at West Plains, Mo., for training and to talk about safety with company executives.

Myers said about 65 percent of the company's flights are from hospital to hospital. For arrivals at the scene of medical emergencies, the company has developed pre-designated sites where a helicopter can land safely, he said.

"You see fire departments in rural areas with a wind sock. They have a landing pad," Myers said.

Myers said the company will learn all it can from Tuesday's tragedy in an effort to make future flights safer.