Investors in a deep sea treasure hunt are opposing a request by the man who led the expedition to be freed from jail on contempt-of-court charges.

Judge Algenon Marbley has held defendant Tommy Thompson in contempt of court since December 2015 for violating terms of a plea deal by refusing to respond to questions about the location of 500 missing gold coins.

Thompson's attorney argues that federal law prohibits holding someone under such conditions for longer than 18 months.

Attorneys for the Dispatch Printing Company, an expedition investor, said in a Tuesday court filing that Thompson should be held indefinitely until he helps find the coins.

The coins, valued at up to $4 million, were minted from gold taken from the S.S. Central America, which sank in an 1857 hurricane.