Odyssey Marine Exploration must give Spain back the 594,000 silver and gold coins it took in 2007 from a 19th century Spanish shipwreck because the property is protected, among other things, by a 1907 US treaty Washington signed with Madrid, a US appeals court said on Wednesday.
In a 60-page ruling, the 11th US Circuit Court of Appeals upheld a lower federal court's decision in December 2009 that granted the treasure to the Spanish government.
Odyssey has said that it will take the fight to the US Supreme Court to keep the Spanish coins it salvaged from the Nuestra Señora de las Mercedes if the federal appeals court in Atlanta ruled against it.
But legal analysts say that there is no clear guarantee that the justices would hear Odyssey's appeal because the Supreme Court only selects a small very percentage of the hundreds of petitions it receives on an annual basis.
The coins have been in the possession of the US Marshal Service during the legal battle.
In the decision, the federal appeals court states that "releasing the [treasure] to Odyssey rather than Spain would be inconsistent with Spain's rights under the 1902 Treaty of Friendship and General relations between the United States of America and Spain."
The Peruvian government also joined the legal battle for the coins because it claimed that they were minted there when Peru was a Spanish colony.
"Because the cargo aboard the Mercedes is treated as part of the shipwreck of the Mercedes for sovereign immunity purposes, the Mercedes' immunity precludes Peru's attempt to institute an action in United States courts against any part of the Mercedes or any cargo it was carrying when it sank," Wednesday's ruling stated.
Twenty-five survivors of the passengers on board the ship when it sank also filed a counter claim, which the US Court of Appeals has rejected.