Detectives searched his home and were seen leaving the property with a plastic box, a ladder and two bags.
"It shows the difference between the two legal systems," Maillaud said. "In France being arrested doesn't mean you are guilty - it's just our way of questioning him. This arrest is not as the result of any new evidence, but is because the British forces now think they have enough evidence to question him."
Maillaud said that it is likely French police would travel to Surrey to speak to their British counterparts about the questioning of al-Hilli.
The prosecutor also said yesterday that they had, for the first time, been able to speak to the daughters of the couple. Zainab, 7, and Zeena, 4, survived the killing of their parents, grandmother and French cyclist Sylvain Mollier near Annecy on September 5. The family were murdered in a remote car park above the village of Chevaline.
"The discussions with the youngest girl didn't really bring anything new," said Maillaud. "But the older girl could confirm that they went to Chevaline to go for a walk. That's her recollection - even if she wasn't exactly aware of what was going through her father's head."
Both girls are in the care of social services.
Zaid Al-Hilli was interviewed, but not arrested, by French police in March, when he is understood to have been asked about an alleged dispute with his brother over a family inheritance.
Their Iraqi father Kadhim Al-Hilli died in Spain two years ago, leaving several properties and £800,000 ($1.6 million) in a Geneva bank account. Saad hired lawyers to block Kadhim's will until "unknown" disputes had been resolved, according to legal papers.
A Swiss prosecutor has claimed that the brothers had been locked in an "inheritance war".
The French investigators are understood to have asked Zaid about allegations that he tried to withdraw cash from the Geneva account using an expired credit card shortly before the murders.
Family tragedy
The victims:
• Saad al-Hilli - 50, British-Iraqi engineer whose family was savagely attacked by a gunman on a forest track near Lake Annecy, France, in September last year.
• Iqbal al-Hilli - 47, wife of Saad. They lived in Surrey, UK.
• Suhaila al-Allaf - 74, mother of Iqbal, who lived in Sweden.
• Sylvain Mollier - 45, a French cyclist who is believed to have stumbled upon the murder scene in the remote forest layby near the village of Chevaline.
The survivors:
• Zeena al-Hilli - 4, was discovered under her mother Iqbal's body eight hours later.
• Zainab al-Hilli - 7, had serious head injuries after being shot and beaten.
The suspect:
• Zaid al-Hilli - 54, brother of Saad arrested on suspicion of conspiracy to commit murder after officers uncovered evidence to suggest he doctored documents to ensure he inherited the family estate.
