The pilots of the airplane were instructed to expect a clearance altitude of above 5,000 feet, according to the National Transportation Safety Board report.
The last radar showed the Learjet 35A at 2,300 feet. All five people aboard were killed in the crash just after midnight Oct. 24 near the U.S.-Mexico border.
``The low altitude warning alerted over the Minimum Safe Altitude Warning system during the flight's last two radar returns,'' the report states.
The report contains preliminary information and is subject to change as the investigation progresses, according to the NTSB.
Killed in the crash were pilot Karl A. Kolb, 56; co-pilot K. John Lamphere, 30; nurse Laura A. Womble, 47, and her husband, Donald, 45, a paramedic, all of Albuquerque; and paramedic Marco E. Villalobos, 33, of El Paso, Texas.
The Learjet belonging to Albuquerque-based Med Flight Air Ambulance Inc. struck Otay Mountain shortly after taking off from Brown Field in clear weather.
The crew was returning to Albuquerque after picking up a patient on a cruise ship off the coast of Mexico and dropping the person off in San Diego.
The plane was one minute into the flight when radio contact was lost.
The cockpit voice recorder has been recovered from the wreckage and sent to the NTSB's Vehicle Recorder Laboratory in Washington for analysis.
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