More than half of gamblers say they still wager using offshore sites like Bovada, MyBookie and BetOnline, according to a survey by the association. The challenge for the gaming industry has escalated as online searches for offshore sportsbooks last year rose faster than searches for the regulated operators, according to the AGA. The Justice Department has not yet responded to the AGA's letter or to CNBC's request for comment. Then on June 29, more than two dozen members of Congress also sent a letter calling on the Justice Department to investigate and prosecute illegal offshore sportsbooks.

In a letter to Attorney General Merrick Garland this spring, the AGA asked the Justice Department to investigate well-known offshore gambling sites, which it said are openly violating federal and state laws and brazenly paying for advertising that targets U.S. 'What maybe at one point in time was a relative nuisance, is now becoming a serious threat to the legal, licensed gaming industry,' Bill Miller, CEO of the American Gaming Association, told CNBC in a recent interview. Personal Loans for 670 Credit Score or Lower Personal Loans for 580 Credit Score or Lower Best Debt Consolidation Loans for Bad Credit

Are us betting sites legal