Investigators on Tuesday examined the wreckage of a sightseeing helicopter that crashed and burned near Great Smoky Mountains National Park in eastern Tennessee, but they said it was too early to tell what caused it to go down.

Luke Schiada, a senior investigator with the National Transportation Safety Board, said there was evidence that the Bell 206 helicopter made contact with the top of a ridge on the side of a mountain when it crashed Monday afternoon, killing four passengers and the pilot. Officials said the touristhelicopter, which was built in 1977 and operated by Smoky Mountain Helicopters, was destroyed by fire after the crash.

Authorities have not released the names of the dead.

Schiada said the tour route that the helicopter was on indicated that it was on a 12-minute flight, and it was the second flight of the day for the pilot in the helicopter. The NTSB said it would be reviewing how the helicopter was loaded, the aircraft's maintenance records, the pilot's background and the wind conditions at the time it crashed, he said.

"The fact that the wreckage was consumed by the fire does make things more complicated," Schiada said at a news conference.

The NTSB will present a preliminary report on the facts of the crash on its website by the end of next week, Schiada said. An investigative report containing the probable cause of the crash could take a year or more, he said, adding the investigation is a "methodical process."

The crash site is less than a mile from a large outlet mall in Sevierville and adjacent to a neighborhood off the main tourist drag. The site is about 3 miles from Dolly Parton's Dollywood theme park. No one on the ground was hurt.

Shawn Matern said he was inside his parents' house Monday when he heard a loud boom. "That's when we came out and saw the second explosion right before our eyes," he said.

He said he saw the pilot roll out of the burning helicopter on the ground and a neighbor went to try to help.

Matern said the tour helicopters fly over at least three or four times a day in that area.

A man who answered the phone at Smoky Mountain Helicopters declined comment and hung up.

Gary C. Robb, a Kansas City attorney who wrote a book on helicopter crash litigation, said it was far too early to determine the cause of the Sevierville crash, but some helicopter tour operators have been known to be reckless to "thrill the tourists" by flying too close to trees or waterfalls or by dangerous maneuvers.