Rupert Wace, a well-known dealer in antiquities in London, and his partner, Sam Fogg, have said they acquired the work legally from the Montreal museum’s insurance company. But after investigators laid out a lengthy and detailed timeline they said showed that the item was in fact stolen, the men signed court papers agreeing to the surrender.

At the time of the seizure, Mr. Wace had said in an email, “This work of art has been well known to scholars and has a history that spans almost 70 years.” He had added that he and Mr. Fogg were “simply flabbergasted at what has occurred.”

The bas-relief is an eight-inch-square piece of carved limestone that was part of a long line of soldiers depicted on a balustrade at the central building on the Persepolis site. It dates to the Achaemenid dynasty — or the First Persian Empire — and experts said it was made sometime between 510 and 330 B.C., when Persepolis was sacked by Alexander the Great.

The district attorney’s office has made investigating looted antiquities a priority in recent years and even created a new squad devoted to that mission last year. Aided by forensic researchers and legal specialists, the office has pieced together the histories of dozens of illicit items that arrived in New York for sale.