PARIS — An engine failure at takeoff led to Monday's deadly crash of the Greek Air Force F-16, which killed 11 personnel at a NATO air base in Spain, French air chief Gen. Denis Mercier said on Thursday.

"What we know is that the F-16 crew, fairly quickly, right after the takeoff, tried to eject," Mercier told journalists. Nine of the personnel lost were French, while the two Greek pilots lost their lives.

"It is an incredibly improbable accident to have an aircraft that has an engine breakdown at takeoff, flies sharply off course and falls right on top of aircraft which are getting ready to fly, so are full of fuel," he said. "It is a series of unlucky events."

No details were given of the cause of engine failure.

The French Air Force will continue to send pilots to Albacete air base, which operates as a NATO training site, he said.

Mercier said he knew the air base well, and that there was not a safety or organizational problem. "It was completely bad luck," he said.

It was such an extremely rare accident and one that led to the fighter hitting aircraft lined up for taking off, he said.

"It was completely bad luck," he said. "A few meters further and there would have been a different outcome." he said.

Mercier said spoke of luck and bad luck, and told how one of the French pilots was sitting in one of the Alpha jet trainers that Monday, studying the flight plans, while the pilot due to fly with him had left his plans by the runway and was not in the plane when the F-16 hit the flight line.

"One is luck, the other enormously bad luck," he said. There was not much more to be done on boosting safety, he said.

A crash inquiry has been launched and the F-16's black boxes have been recovered, he said.

The Air Force will hold Monday as a day of mourning on Monday, for the service, while President François Hollande has set Tuesday as a national day of mourning, with an official ceremony at Les Invalides, the military museum here.

The bodies of the nine French personnel are today being flown back to Nancy air base, eastern France, on Thursday, the Air Force said in a statement.

Among the aircraft hit by the F-16 were two French Alpha jets and two Mirage 2000-D fighters.

The pilots were taking part in advanced multinational training in the NATO Tactical Leadership Program, based at the Spanish air base.

Dassault Aviation offered sincere "condolences to the relatives of the victims, assures the injured of its support and confirms its solidarity with the close-knit family that is the Air Force," the company said in a Jan. 27 statement on Tuesday. Dassault built the Alpha jets and Mirages.

"These aviators died or were wounded while in the conduct of their mission," the chief of the defense staff, Army Gen. Pierre de Villiers, said in a Jan. 28 statement on Wednesday. "They were training to face up to the most demanding operations the Air Force undertakes everywhere in the front line." he said.