Saad al-Hilli and wife Ikbal, from Claygate, Surrey, and her mother Suhaila al-Allaf, who lived in Sweden, were all shot dead on a remote forest road in Chevaline, near Annecy, on September 5, 2012. Local cyclist Sylvain Mollier was also murdered.
French police arrested a 48-year-old, believed to be a former policeman from the Haute-Savoie region of France, on Tuesday in connection with the killings.
Annecy prosecutor Eric Maillaud said yesterday that 40 wartime guns - but not the murder weapon - were found when officers searched the suspect's home and that of his in-laws.
The man was arrested after an e-fit was released of a motorcyclist spotted near the scene, but the unusual helmet the rider wore was also not found.
Mr Maillaud said police arrested a second man who was suspected of illegally dealing in firearms. He said: "The investigators found a large number of weapons, also grenades and a shell."
Last month Mr al-Hilli's brother Zaid al-Hilli, who was arrested in connection with the shooting, had his bail cancelled by Surrey Police after the force decided there was not enough evidence to charge him with a crime.