(CNN) Casino magnate Sheldon Adelson and his wife Miriam Adelson contributed $500,000 to the outside legal fund that's helping foot the bill for current and former Trump administration and campaign aides embroiled in the Russia investigation, according to a recent filing with the Internal Revenue Service.

The Patriot Legal Expenses Fund Trust was set up last year raise money from donors to pay legal expenses for multiple individuals.

It's unclear which officials are getting support from the legal fund, which has declined to disclose its beneficiaries. But the fund's draft agreement made clear that it can't cover President Donald Trump's legal bills.

The latest IRS filing lists a series of payments made to several law firms: Aegis Law Group; Berliner, Corcoran & Rowe; Jones Day; King & Spalding; and Mintz, Levin, Cohn, Ferris, Glovsky & Popeo.

The Adelsons' contribution is the largest to date, but follows six-figure contributions from other Trump friends and GOP megadonors. Mar-a-Lago member Anthony Lomangino has previously contributed $150,000, real estate developer Geoffrey Palmer has given $100,000, and casino mogul and Trump friend Phil Ruffin has contributed $50,000.

The Adelsons have been among the most prolific individual donors to Republican candidates for years. This past midterm election cycle, the two were among the GOP's largest benefactors, donating a whopping $112 million to Republican super PACs through September.

To date, the Mueller investigation has yielded charges against 37 people and entities . The defense legal funds have been costly for those swept up in the Mueller investigation -- including those merely called up for questioning -- with some having to pay close to a million dollars in legal fees.

Roger Stone, who was indicted last week, told CNBC in April that his legal fees were already adding up to more than half a million dollars. In September, around the time of his guilty plea, former Trump campaign chairman Paul Manafort's legal fees mounted to more than $1 million, according to two people familiar with his case.