NORTH LAS VEGAS, Nev. (AP) - Authorities were investigating today the cause of a single-engine plane crash that killed six people shortly after takeoff on Christmas Day.

The burned wreckage of the Beech A-36 remained in a concrete wash on the southern edge of the North Las Vegas Airport, where a National Transportation Board investigator said it would probably be next week before an initial report on the crash will be released.

``There is a lot of burn damage. It's been difficult,'' said NTSB investigator Howard Plagens.

The bodies of four adults and two children were removed from the wreckage Thursday evening. A spokeswoman with the Clark County coroner's office said today the bodies had not yet been identified.

Officials with the Federal Aviation Administration said the six-seat aircraft took off at 1:21 p.m. but crashed moments later -- after the pilot radioed an emergency and indicated he would try to return to the airport.

Plagens said the pilot reported engine trouble.

The plane crashed about 400 yards south of the runway, on airport property about 10 miles northwest of the Las Vegas Strip. There were no homes or businesses in the immediate area.

Plagens said the plane owner moved it about a month ago to the North Las Vegas Airport, a busy Clark County facility that primarily serves private, corporate and commercial sightseeing flights.

Thursday's crash was the third at the airport in recent months. On Aug. 31, two people received minor injuries when their Piper Malibu crashed short of a runway. In September, two pilots were injured in a runway collision.

The airport and its three runways handled more than 218,000 takeoffs and landings last year. It is the second-busiest airport in Nevada, behind McCarran International Airport in Las Vegas, and is one of the 54 busiest in the nation.

In recent years, it upgraded its runway, layout and lighting, and a new control tower opened in early 2002.