Before Donald Trump's phone call with his Ukrainian counterpart sparked the whistleblower complaint that might lead to impeachment, another whistleblower had filed a different complaint, this one related to the president’s tax returns.

Richard Neal, a Democrat representing Massachusetts in the House of Representatives, said the House ways and means committee is looking to interview a whistleblower who alleges “inappropriate attempts to influence” a tax audit of Mr Trump, according to the radio station WBUR.

Mr Neal sent a letter to treasury secretary Steven Mnuchin on August 8 saying the committee, which he chairs, received an “unsolicited communication” from a federal employee on July 29.

Mr Neal called the allegations “credible” in his letter to Mr Mnuchin, saying they pointed to “evidence of possible misconduct.”

"This is a grave charge that appreciably heightens the committee's concerns about the absence of appropriate safeguards as part of the mandatory audit programme and whether statutory codification of such programme or other remedial, legislative measures are warranted," he wrote in the letter.

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He also said that his committee had "raised these concerns repeatedly”. Charles Rettig, the IRS commissioner, has previously dismissed the claims.

In July, Mr Neal sued Mr Mnuchin and Mr Rettig in to obtain six years of Mr Trump’s tax returns, which the president has notoriously refused to release.

Mr Neal says he’s unsure how the whistleblower complaint might affect this lawsuit.

"It's hard to say at this point,” he said at an appearance in Springfield, Massachusetts on Tuesday, “because as it makes its way through the federal court system, there will be opportunities to, I think, make that assessment, perhaps a little bit later.”

He said it’s up to the House legal counsel to decide whether or not to release this whistleblower complaint.

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Asked about whether or not this complaint might intersect with the current impeachment inquiry, which continues to move forward following a separate complaint, Mr Neal said it was “hard to speculate about”.