MALIBU A federal investigation is underway to determine the cause of a plane crash that occurred here shortly after the pilot left the Santa Monica Airport on Tuesday evening.

A Sky Arrow 600 Sport dove into the ocean just off Malibu Pier at approximately 5:15 p.m., critically injuring its pilot and passenger, who were airlifted to the UCLA Medical Center in Westwood.

Information about the two crash victims has not been released. The Federal Aviation Administration and the National Transportation Safety Board are conducting an investigation.

FAA spokesman Ian Gregor said the plane crashed about 50 yards offshore near the intersection of Pacific Coast Highway and Malibu Canyon Road. He believes the pilot was flying the plane locally and was expected to arrive back at SMO.

The plane is categorized as a Light Sport Aircraft, a classification that the FAA established just four years ago for single-engines that carry one or two people, weighing no more than 1,320 pounds and flying at a maximum speed of 120 mph.

The plane was registered to Northfield Aviation LLC, which has a commercial permit to give flight instruction and rent planes, according to Bob Trimborn, the director of Santa Monica Airport. The small aircraft was based on the north side of the airport.

A representative with Northfield Aviation LLC said the company has no comment regarding the crash.

melodyh@smdp.com