A Dutch art detective has claimed that someone left an ancient relic with him last week after it was stolen in France last month.

His doorbell rang, and when he opened the door no one was there. Instead, a cardboard box was waiting for him at the door in the dark.

In that box he said was one of the most sacred artifacts ever stolen from the Catholic Church: the "Precious Blood of Christ" relic.

"(My) heart was beating in (my) chest," the Dutch art detective told Agence France-Presse (AFP) in an interview about the recovery of the stolen treasure.

The brand is dubbed the "Indiana Jones of the Art World" for his remarkable recovery of stolen art, including the "Hitler's Horses" bronze statues, a Picasso painting, and a ring that once belonged to Oscar Wilde.

At 52, he is one of the most famous art sleuths in the world, trusted by both thieves and police for his unprecedented access to the criminal trade in stolen art.

But getting his hands on the "Precious Blood of Christ" relic was a particularly special experience for him.

"As a Catholic myself, this is about as close to Jesus and the legend of the Holy Grail you can get."

"It was a religious experience," he said.

When he opened the box, he discovered what he hoped he would: the relic was perfectly intact.

The ornate container, about 30 centimeters (11 inches) high, has two metal bulbs inside and is said to hold drops of Jesus' blood collected in the Holy Grail during the crucifixion.

It is held in a heavily gilded copper box with encrusted precious stones and depictions of Christ on the cross and other saints.

The holy relic is believed to date back to the death of Christ and has been a source of worship for Catholic pilgrims for more than 1,000 years.

It was held by the Fecamp Abbey in France's Normandy region until the night of June 1-2, when it was stolen, just two weeks before the annual "Mass of the Precious Blood" celebration.

"The thief most likely got inside by being locked in after-hours and made off with the artifact," Brand said.

"It was a massive shock that this famous piece, this legendary piece was stolen," he told AFP in his home, the relic on display nearby.

The relic and its copper box were not the only objects waiting in the cardboard box on Brand's doorstep on that fateful July evening.

There were several copper liturgical plates, depictions of saints, and an ornate goblet, which were also stolen from the abbey in June.

The brand carefully showed AFP the ornate and heavy copper box with a miniature roof and four angels on its corners called a reliquary.

The brand will hand the "Precious Blood" artifact to Dutch police Tuesday, who will give it to French authorities to be eventually returned to the Fecamp Abbey.

AFP could not independently verify the authenticity of the objects, and they have not yet been examined by police or experts. The abbey could not immediately be reached for comment.

But Brand does not doubt its authenticity.

"I do not doubt in my mind that it is the real thing. Religious objects are almost impossible to forge."

Brand said his involvement in the case started a few days after the theft, when he received a protected email from an anonymous writer, claiming to have the stolen loot in their possession.

"This person was approaching me on behalf of another, at whose home the stolen relics were being stored," said Brand.

But "to have the ultimate relic, the blood of Jesus in your home, stolen, that's a curse," he said.

"When they realized what it was, that you in fact cannot sell it, they knew they had to get rid of it."

The brand showed AFP an email written in Dutch in which the person asked him to take back the stolen property, as it was too risky to return it to the abbey itself.

The brand was told that the stolen artifact would be delivered to his home, but no date or time was given.

"I was virtually a prisoner in my own home for a week. I could not leave," he said, laughing.

Finally, the objects turned up, delivered by an anonymous sender.

Asked why the thieves would return the stolen art to Brand, the detective said "it would be too dangerous to involve the police."

"These people know my reputation and that the most important thing is to return this to the church. Hopefully, it will stand for another thousand years," Brand said.