ANKARA, Turkey (AP) - U.S. experts on Sunday joined the investigation into what caused a plane crash that killed 57 people in southwest Turkey.

The Atlasjet MD-83 was flying from Istanbul to Isparta in Turkey's southwest when it crashed into a mountain early Friday, killing the crew of seven and all 50 passengers, including a group of physicists working on a national nuclear project.

The cause remained unclear Sunday but Turkish authorities have ruled out sabotage.

A team from Boeing - which acquired the aircraft manufacturer McDonnell Douglas - and from jet engine manufacturer Pratt & Whitney visited the crash site Sunday and met with a Turkish prosecutor leading the investigation, the Dogan news agency reported.

A Transport Ministry official confirmed that U.S. experts were in Turkey. He asked not to be named because he was not authorized to speak to the media.

An initial study of the crash site indicated that the pilot may have veered several miles off course while making the nighttime landing, news reports said Sunday.

The Transport Ministry official refused to confirm the reports, saying the exact cause would be clearer when the flight data and cockpit voice recorders are examined.

Weather conditions and visibility were good when the plane disappeared from the radar a few minutes before the scheduled landing. A transcript of the conversation between the Atlasjet pilot and the Isparta control tower, released by Dogan, did not indicate any problem.

Funerals were held over the weekend in Isparta, the Aegean port city of Izmir and in Istanbul.

Atlasjet, a private airline established in 2001, operates regular flights inside Turkey and chartered flights to Europe and other foreign destinations.