A Swiss F/A-18 warplane smashed into the Alps after flight controllers told the pilot to fly far below a safe altitude, investigators probing the mystery incident said Tuesday.

The advanced fighter jet disappeared over the central Swiss Alps on August 29. Its wreckage was found the following day in the remote Hinter-Tierberg region at an altitude of around 3,300 metres (11,000 feet).

Military flight controllers at Meiringen base "gave the pilot altitude data that was too low for the specified area," military judicial investigators said in a statement.

Flight control had given the "floor" altitude -- the minimum required for safe passage through the area -- at 10,000 feet, when it should have been 14,300 feet, they said.

The pilot, a 27-year-old career officer in the Swiss armed forces, had taken off with another F/A-18 single-seater for a joint exercise with an F-5 fighter.

The crash happened amid thick cloud, the statement said, adding that the ejector seat "appears not to have been activated."

The investigation is continuing, it said.