VALLETTA/PARIS (Reuters) - A small aircraft conducting a surveillance mission over the Mediterranean for France’s defense ministry crashed on take-off in Malta on Monday, killing all five people on board.

The ministry declined to say what the purpose of the surveillance operation was. Airport officials said the plane had been heading for Misrata in Libya, where some Western powers have sent small teams of special forces to support the new U.N.-backed unity government in its fight against Islamist militants.

The twin-prop Fairchild Metroliner went down near the runway of the southern Mediterranean island nation’s main airport in the morning, sending smoke billowing into the sky. Malta International Airport was closed for several hours.

On board were three French defense ministry officials and two pilots, who were also French. France’s defense ministry and Luxembourg-based CAE Aviation, which operated the plane, said the five died. CAE said it was an accident but gave no details.

“A light reconnaissance aircraft ... carrying out surveillance operations over the Mediterranean for the defense ministry crashed this morning at 0630 at Malta’s Luqa airport, during take-off,” Defence Minister Jean-Yves Le Drian said.

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The French daily Le Monde wrote that the defense ministry officials belonged to France’s intelligence services.

The Maltese government initially said the flight was part of a customs operation France has been conducting for the last five months, tracking human trafficking and drug smuggling. But French customs said they had no personnel on board.

The remains of all five victims were found, the Maltese government said, and inquiries were under way to determine what had happened. “Official information, footage and eyewitnesses ... clearly indicate that there was no explosion prior to impact,” it said in a statement.

The plane, registered in the United States and leased to CAE, “was being flown by an experienced crew with no technical issues reported on previous flights”, a CAE statement said. The cause was unknown but was being investigated.

The flight had been due to return to Malta within hours without landing in any other countries, Malta’s government said. The crash was the country’s worst peacetime air incident.