The White House has told a US congressional committee it will not comply with a request to release files related to a controversial former aide.
The House Oversight Committee is looking into payments to fired National Security Adviser Michael Flynn.
Lawmakers suspect he failed to disclose money he received in December 2015 to address a Moscow gala, where he sat next to President Vladimir Putin.
Mr Flynn quit in February after misleading the White House.
The retired US Army lieutenant general had failed to divulge his conversations with the Russian ambassador, to Vice-President Mike Pence.
Mr Flynn's links to Russia are being scrutinised by the FBI and two congressional committees, as part of wider investigations into claims that Moscow sought to help Donald Trump win the US presidential election.
In addition to those inquiries, the House Oversight Committee is looking into whether Mr Flynn fully disclosed payments from Russian and Turkish or other foreign sources.
The oversight panel wanted to see his application for security clearance before he joined the White House administration in January.
But the White House referred the request to the defence department.
White House legislative affairs director Marc Short said in a letter on 19 April that he would be "unable to accommodate" the request because it relates to Mr Flynn's actions before joining the White House.
Massive vetting breakdown - Anthony Zurcher, BBC News, Washington
The legal noose appears to be tightening around Michael Flynn's neck.
While he was forced to resign as Donald Trump's national security adviser because of political concerns - what the White House said was a lack of honesty with Mr Pence and other administration officials - he's now being accused of disclosure violations that could carry criminal charges.
While this is obviously concerning to Mr Flynn, it's also a significant embarrassment for the White House, as these revelations represent a massive breakdown in the vetting process for its high-ranking officials.
The position of national security adviser is extremely sensitive - effectively the president's eyes and ears within the defence and intelligence communities - and the person who fills the role should be free from any possible foreign entanglements or possible ethical conflicts.
The contours of Mr Flynn's Russian connections were firmly in view as he emerged as a contender for a White House job and yet, it appears, the president and his team either pressed on without delving deeper into the matter or - perhaps more concerning - allowed the former general to come on board in spite of everything they may have known.
Such actions border on gross negligence.
Follow @awzurcher on Twitter
The Republican chairman of the oversight committee, Jason Chaffetz, said Mr Flynn did not report payments for the 2015 Russia trip on his top-level security clearance form.
"I see no data to support the notion that General Flynn complied with the law," said Mr Chaffetz.
Mr Flynn received more than $33,000 (£25,700) from Russian broadcaster RT for giving a speech at its Moscow gala. He was seated at a table next to the Russian president.
Mr Flynn did not initially disclose payments received by foreign governments when filing financial disclosure forms related to his White House security clearance.
He filed an amended form in March, revealing three payments from the Russian government.
He also received more than $500,000 from Turkey for lobbying he undertook on their behalf in Washington after leaving the Defense Intelligence Agency in August 2014, and before joining the Trump administration.
Mr Flynn wants immunity to testify before Congress on alleged Russian election meddling, his lawyer has said.
"As a former military officer, you simply cannot take money from Russia, Turkey or anybody else," Mr Chaffetz said.
"And it appears as if he did take that money. It was inappropriate.
"And there are repercussions for the violation of law."