French authorities discovered a weapons stash and detained a second man over the shooting deaths of a British-Iraqi man and three other people 18 months ago in the Alps, a state prosecutor said Wednesday.

Police sweeps are continuing over the killings of Saad al-Hilli, his wife, an elderly relative and a French cyclist in a wooded area near bucolic Lake Annecy in September 2012, said prosecutor Eric Maillaud.

"This case has not been solved," he told a news conference.

A 48-year-old security guard and former municipal police officer who was detained on Tuesday was the first person apprehended in France over the killings, highlighting the complexities of a case with faraway ties to Iraq and Britain.

Grenades, mortars found

Police sweeps turned up vintage guns, a grenade and mortars in his home and that of a friend also detained for questioning, Maillaud said. Authorities are now considering a separate probe into alleged weapons trafficking or sales linked to the men, he said.

The prosecutor provided new details about the former officer — whom he declined to call a suspect — calling him a weapons buff who had once threatened tourists and shown sporadic bouts of violence. Unidentified neighbours had considered him a racist, he said.

The men were not identified because no link between them and the killings had been established, Maillaud said. But he said one weapon in the ex-officer's stash was made by Luger — the same manufacturer as the murder weapon, if not the same calibre or model, Maillaud said.

The al-Hilli couple's two daughters survived the shooting, with the younger one huddled beneath her mother's lifeless body in the car.

In June, British police detained al-Hilli's brother, Zaid, for questioning and later freed him on bail.