Investigators hunting for one of France’s most notorious killers were stymied this week when their chase led them to a monastery — where most potential witnesses have taken a vow of silence.

Someone at the monastery — home of the reclusive Carmelite brethren — on Tuesday reported a man resembling Xavier Dupont de Ligonnes, a French aristocrat who has been on the lam since 2011 for the murder of his wife, four children and their two Labradors.

Dozens of armed officers surrounded the religious site in the medieval village Roquebrune-sur-Argens in southern France, where Dupont de Ligonnes was last seen seven years ago.

But investigators struggled to communicate with the monks, and a two-hour search of the monastery yielded no trace of the alleged killer, investigators told AFP.

“We believe worshippers in fact saw a monk resembling Dupont de Ligonnès,” a police source told Telegraph UK.

The 57-year-old businessman, who is the descendant of a count, is suspected of slaying and burying his family under the terrace of their elegant Nantes townhouse seven years ago.

Police said the high-class creep shot his wife, Agnes, 49, daughter Anne, 16, and sons aged 13 to 20 in a “methodical execution” with a .22 caliber firearm.

They were missing for three weeks before their bodies were discovered.

Dupont de Ligonnes reportedly told the Catholic school his kids attended that the family was moving to Australia and told friends he was a secret agent being taken into witness protection.

Since he was last seen leaving a budget hotel in the village 50 miles from Cannes, police have received thousands of calls reporting sightings of him, all of which turned out to be false leads.