The Florida treasure-hunting firm battling Spain in the U.S. courts for possession of $500 million in gold and silver coins salvaged from the Atlantic in May 2007 tried to aid its case by getting Congress to alter the wording of a law, the Washington Post reported.

Working through sympathetic Florida lawmakers and a Washington lobbying firm, Odyssey Marine Exploration, Inc. sought to insert minor changes in the 2004 Sunken Military Craft Act.

The changes were to have been introduced via a defense appropriations bill, but House and Senate negotiators decided this week to leave the proposed language out of the legislation.

U.S. District Judge Steven D. Merryday ruled in December 2009 that Spain was the rightful owner of the treasure Odyssey salvaged off the Portuguese coast in the same area where the Nuestra Señora de las Mercedes, a Spanish navy frigate, was destroyed in battle in 1804.

Merryday's ruling was upheld last month by the 11th U.S. Circuit Court of Appeals, but Odyssey asked the court to stay its order to hand over the treasure pending an appeal to the Supreme Court.

Madrid says the treasure came from the Mercedes and that the vessel and its contents rightfully belong to Spain under the principle of sovereign immunity.

Odyssey, however, contends that contemporaneous Spanish diplomatic communications show the Mercedes was on a commercial mission at the time of her sinking, invalidating Madrid's sovereign immunity claim.

Within days of recovering the $500 million in coins, Odyssey took the hoard to Gibraltar and loaded it onto a chartered Boeing-757 for transport back to Florida.

The treasure remains at a secret location in Florida.

The alterations Odyssey sought to U.S. legislation could conceivable have bolstered that argument, but a source at the Spanish Embassy in Washington told Efe the firm's hopes were misplaced.

As written, the 2004 law defines "sunken military craft" as "any sunken warship, naval auxiliary, or other vessel that was owned or operated by a government on military noncommercial service when it sank."

Even if the wording of the law were changed, Odyssey would still lose in court, according to the Spanish Embassy source, because "a military ship is military by its nature, and cannot be "military commercial.'"

Moreover, the source said, it was common practice for military vessels to carry merchandise and even civilian passengers, while documents show the Mercedes was sailing to Spain on the orders of the Spanish king.