Washington (CNN) The top Democrat on the Senate Finance Committee is dialing back his threat to block Treasury Department nominees, saying he will allow hearings on three nominees to proceed.

In a statement, Oregon Sen. Ron Wyden said Thursday the Treasury Department has "finally cooperated" with his investigation into its handling of a request by House Democrats for six years of President Donald Trump' s personal tax returns.

Wyden said the July 17 letter by Justin Sok , senior adviser for Treasury's office of legislative affairs, "acknowledged the unprecedented process that led to its refusal to comply with a lawful request."

In its letter, Wyden said, the agency admitted no Treasury secretary to their knowledge had ever been previously involved in responding to a tax return request under an obscure tax law, known as Section 6103, as Secretary Steven Mnuchin has been involved in handling the response to the request for Trump's returns.

The letter also confirmed that the Office of Legal Counsel had never been formally consulted previously on such a request, and that Treasury had never formally considered whether there was a legitimate legislative purpose behind the tax law.

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