A rare British Guiana stamp that was previously owned by convicted killer John duPont, set the auction record for collectible stamps, selling for $9.5 million on Tuesday.

The 1856 stamp is just one-inch by one-and-a-quarter inch and is printed on magenta paper. Sotheby’s in New York said that it is the most expensive stamp sold at auction in history, notes Reuters, easily surpassing the previous record. It is the only known stamp of its kind remaining.

Back in 1996, the famous Treskilling Yellow stamp set the previous record, selling for $2.2 million. That 1855 stamp was printed in the wrong color.

The new owner of the British Guiana stamp has remained anonymous, calling the winning bid in over the phone.

It was previously owned by duPont, the heir to the chemical fortune, who has a stamp collector. He paid $935,000 for it in 1980, notes the Washington Post. He was convicted of murdering David Schultz and died in prison in 2010. His story is told in the upcoming film Foxcatcher, with Steve Carell playing duPont.

Sotheby’s was actually hoping that the stamp would sell for more, estimating that it would go for as much as $20 million. Still, Sotheby’s vice-chairman David Redden said that they were happy with the $9.5 million result.

“That price will be hard to beat, and likely won’t be exceeded unless the British Guiana comes up for sale” again, Redden said.