The ceremonial ships that the Roman Emperor Caligula built to host decadent festivities on Lake Nemi were ornate floating palaces, with pink marble columns and brightly colored mosaic floors. Adorned with gold and gems and bronze friezes of animals, they were the sites of mega-parties that sometimes lasted days, according to historical accounts.

But for much of the past five decades, a four-by-four piece of mosaic flooring from one of the ships has been sitting in a somewhat more prosaic setting, the Park Avenue apartment of an antiques dealer, where it was used as a coffee table, often to hold a vase of flowers and, occasionally, someone’s drinking glass.

Now investigators for the Manhattan district attorney’s office are trying to sort out the journey of the 2,000-year-old piece of Roman history that was once dredged from the lake outside Rome and somehow ended up in a private home in New York City. Last month, prosecutors seized the mosaic, saying they had evidence it had been taken from an Italian museum before World War II. On Thursday evening, the piece was returned to the Italian government at a ceremony, along with two other recently recovered antiquities.

“These items may be beautiful, storied, and immensely valuable to collectors,” the district attorney, Cyrus R. Vance Jr., said in a statement, “but willfully disregarding the provenance of an item is effectively offering tacit approval of a harmful practice that is, fundamentally, criminal.”