Argentine authorities were on Thursday investigating an apparent mechanical failure on an Argentine air force plane that crashed on the high plains outside La Paz, killing all six people on board. The Learjet 35A was taking part in efforts to aid Bolivian flood victims and was returning from the Bolivian capital to Argentina when it went down, said Argentine air force Commodore Guillermo Lozada. "The plane had taken off from the airport in La Paz on return to our country when, for a failure that will be investigated, it plunged to the ground, killing all of its occupants," Lozada said. The plane crashed in Bolivia's high plains about 30 kilometers southeast of La Paz, Bolivia's capital, according to Col. Johnny Vera of the Bolivian armed forces. Witnesses said they saw the plane on fire in the air and flip before crashing. It apparently crashed five minutes after take off. "The plane completely disintegrated," Vera said.