Industrial espionage is now regarded as one of three possible explanations for the the Al-Hilli killings in the French Alps a year ago this week.

French investigators today put forward for the first time the possibility that Saad Al-Hilli, a satellite engineer, might have been the target of an assassination by a “state agency” connected with technological spying.

But the Annecy prosecutor, Eric Maillaud, admitted to an anniversary press conference that the mystery of the quadruple murder on a forest lay-bay above Lake Annecy in the French Alps remained almost intact.

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Despite 12 months of exhaustive investigations in France and Britain, “there is not the start of the beginning of proof” of who may have been responsible, Mr Maillaud said.

Two other lines of inquiry remained open, he said.

The first was a “violent” quarrel between Saad al-Hilli and his brother Zaid over their father’s €3-5 million inheritance. The second was a targeted hit by an unknown person or persons in Iraq, who wanted to deprive both brothers of their father’s legacy.

On 5 September last year, Saad a-Hilli, 50, a British-Iraqi satellite engineer, his wife Iqbal, 47, a dentist, and her mother Suhaila al-Allaf, 74, were found shot in the head in the family BMW estate two miles from the village of Chevaline. The body of a local cyclist, Sylvain Mollier, 45, lay nearby.

The couple's 7-year-old daughter, Zainab al-Hilli, was found at the scene with head and shoulder injuries. Her sister, Zeena, 4, was found unharmed eight hours later hiding beneath her dead mother’s legs in the back of the car.

In June, British police arrested Mr al-Hilli’s older brother, Zaid, 54, on suspicion of ordering the killings. He was released on bail without charge.

“Zaid should not at all be regarded as the number one suspect,” Mr Maillaud said yesterday. “He had a motive in the sense that he was in a serious conflict with his brother but there is not the shadow of an element pointing to his guilt which could justify keeping him in custody.”

Zaid al-Hilli protests his innocence and denies that there was any quarrel over their father’s will, thought to include property and cash in Britain, Switzerland, Spain and Iraq. He will be questioned again, Mr Maillaud said.

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“People have been killed for less than that,” the prosecutor said. “There was a violent conflict between them. Saad was afraid of his brother.”

The prosecutor – who is no longer leading the investigation but remains its chief spokesman - said the “family quarrel” was only one of three “equally strong” lines of inquiry. Mr Maillaud revealed that Saad al-Hilli had kept at his home in Claygate, Surrey, an “unusual” amount of documents from his work on weather-forecasting and crop-watching micro-satellites.

“His company worked for many foreign states,” Mr Maillaud said. “Any mention of foreign countries and industrial espionage inevitably raises the possibility of the involvement of secret intelligence agencies.”

“This is a very complex part of the inquiry… but investigations are far from being closed on this subject.”

On the third, Iraqi, line of inquiry, Mr Maillaud said the authorities in Baghdad had promised to cooperate but had still not responded to an international warrant issue in November last year.

“The question is whether there is anyone in Iraq who currently controls part of the father’s inheritance who would benefit from the disappearance of both al-Hilli brothers.”

Mr Maillaud insisted that the “very complex” investigation was not “stuck” and had made some progress recently. The US authorities, after months of delay, had finally approved access to data from Saad al-Hilli’s computers, stored across the Atlantic.

The prosecutor also revealed that Mr al-Hilli had recorded all of his phone conversations and investigators were going through the tapes.

There were still 40 French and 40 British investigators working on the case, the prosecutor said. In France alone, 3,000 witnesses had given formal statements and 107 expert reports had been commissioned. Formal requests for information had been sent to 23 countries.

The killer is known to have used an antique 7.65mm Lugar P06 revolver, issued to the Swiss army and police in the 1920s and 1930s. Part of the handle of the gun was found at the scene.

Three clips of eight bullets were fired in a very short time, Mr Maillaud said. There were no bullet impacts on the bodywork of the car. This suggested that the attacker was an “experienced killer” but, otherwise, investigators had been unable to create a clear profile.

The use of such an old gun – widely available locally but not a weapon of choice for professional hitmen or secret agents – is one of the great remaining mysteries of the investigation.

Representatives of Surrey police were also present.

Detective superintendent Nick May said: “We remain committed to finding answers to what happened that day on behalf of their (the victims') families, particularly for the two young girls who lost their parents.

“We have established a good working relationship with our French colleagues and are continuing to pursue a number of lines of inquiry in the UK.”