TAMPA, Fla. - Florida deep-sea explorers who raised an estimated $500 million treasure from the 200-year-old wreck of a Spanish galleon should give all the loot back to Spain, a federal magistrate judge said.

But the two-year tug-of-war over the 17 tons of silver coins and other artifacts from what is believed to be the Nuestra Senora de las Mercedes y las Animas is likely far from over.

Odyssey Marine Exploration said it would oppose Wednesday's written recommendation by U.S. Magistrate Judge Mark Pizzo, which will be considered by another federal judge who will issue an order later.

Odyssey chief executive officer Greg Stemm said yesterday that the company was prepared to keep fighting.

"This case addresses some very significant legal issues," Stemm said, "so in the beginning it became fairly clear it was going to go to the appellate court level."

Pizzo's written recommendation said the wreck was almost certainly that of the Mercedes, a navy galleon that sank in the Atlantic Ocean west of Portugal in 1804. He accepted the Spanish government's argument that it had never expressly surrendered ownership of the ship and its contents.

Odyssey has argued that it still lacks conclusive proof of the ship's identity and disputed the Spanish government's ownership of the valuable cargo.

"We are very happy," Spanish Culture Minister Angeles Gonzalez-Sinde said yesterday. "This decision is very important. I am glad the judge has really seen that the ship and the treasure belong to Spain."

Odyssey created an international stir when it announced in May 2007 that the 500,000 silver coins and other artifacts had been raised from an Atlantic Ocean wreck and flown to Tampa. Spain then went to U.S. District Court claiming ownership of the treasure if it is in any way connected to the country's national heritage.