A Dutch art detective has discovered a pair of Medieval carved reliefs that were stolen from an ancient Spanish church in 2004. The detective, Arthur Brand, found the seventh-century visigothic artifacts at the residence of an unnamed British aristocrat, who used them as ornaments in his garden north of London.

The stone works, thought to be at least 1,000 years old, were returned to the Spanish embassy in London on Monday morning in a private ceremony. The repatriation is the culmination of a 15-year search that started in 2004 when thieves targeted the Santa Maria de Lara church, one of the oldest Catholic chapels in Spain, which was itself only rediscovered in 1921.

Brand received a tip in 2010 informing him that two suspicious artifacts had turned up on the British market that were being offered as garden ornaments, though they clearly were not. The informant eventually led Brand to a French dealer who unwittingly bought them from the Spanish thieves and sold them to a British dealer, who in turn sold them to the most recent owners. The nobleman appears to have bought the works in good faith for around £50,000 each and did not know of their dubious origin, according to AFP.

“The thieves wanted to sell them and make a lot of money, but soon found out they stole world heritage that would be extremely difficult to sell, so they decided to sell them as garden ornaments so they could still make some money,” Brand told AFP. “Once we contacted the owners they got quite nervous, because these priceless 1,300-year-old artifacts that were made for the Spanish sun were in their garden exposed to the English rain.”

Brand said it’s “impossible” to quantify the market value of the artifacts. “There are so few of these things around from the seventh century Visigothic Kingdom,” he said. “There’s not much left, these are crucial things. They made the church a national monument in Spain. It’s quite unique.”

The detective added that only five to 10 percent of stolen art turns up while the rest is typically destroyed, lost, or hidden. “The Spaniards were very, very happy,” he said. “They couldn’t believe it.”