Saad al Hilli, 50, was killed with his wife Iqbal, 47, and mother-in-law Suhaila al Allaf, 74, on a road near Annecy on 5 September 2012.

A gunman sprayed their BMW with bullets at point-blank range in a lay-by.

French cyclist Sylvain Mollier, 45, also died after being shot seven times.

The bodies of the Iraqi-born engineer, his dentist wife, her mother and the cyclist were discovered on a remote forest route.

The al Hillis' young daughters survived.

Four-year-old Zeena hid for hours in the footwell under her dead mother's legs, while her sister, Zainab, seven, was shot in the shoulder and beaten in the head.

The family, from Surrey, were on a camping trip in the country.

In 2013, Eric Maillaud, then prosecutor on the case, concluded: "We are dealing with a very experienced gunman."

The prosecutor in Annecy has now revealed that an arrest was made at 8.05am on Wednesday, while French media reported house searches were taking place and alibis checked.

Police have been unsure whether the al Hillis or Mr Mollier - who worked in the nuclear industry - was the target.

No one has ever been brought to justice over the brutal murders, which prompted a search of the family's home in Claygate, Surrey.

Mr al Hilli's brother, Zaid, was arrested on suspicion of murder in 2013 but released without charge and told he would face no further action.

Another suspect previously arrested in connection with the case was an Iraqi prisoner known as Mr S who was claimed to have said he had been offered "a large sum of money" to kill Iraqis living in the UK.

In 2020, the al Hillis' daughters were set to be re-interviewed, and last year investigators returned to the murder scene, near the village of Chevaline.

Police were also investigating a possible link with a gang of contract killers living in Paris.

One theory suggested Mr al Hilli had been attacked because of his job as an engineer, while another focused on a row over a family will, but no definite motive was established.

A local dispute was considered the most likely and the family were simply and innocently caught up in it.

Other possible reasons probed by authorities have ranged from the family's time in their native Iraq and alleged links to Saddam Hussein, to a "lone-wolf" killing.