President Donald Trump's former national security adviser, Michael Flynn, invoked his Fifth Amendment right against self-incrimination in rebuffing a subpoena Monday in the investigation into Russia's election meddling. Then a top House Democrat cited new evidence he said appeared to show Flynn lied on a security clearance background check.

With Trump himself in the Mideast on his first foreign trip as president, investigations into Russian interference in the 2016 campaign — and allegations of Trump campaign collaboration — showed no sign of slackening in Washington. Flynn's own defensive crouch revealed the high legal stakes he faces as investigations intensify: a U.S. counterintelligence probe of Russia, a federal investigation in Virginia and multiple congressional inquiries.

As well, The Washington Post reported Monday that Trump asked two top intelligence chiefs in March to deny publicly that there had been collusion between the Russians and the Trump campaign.

Citing current and former officials, the Post said the national intelligence director, Daniel Coats, and the director of the National Security Agency, Adm. Michael S. Rogers, both refused Trump's request, judging it inappropriate. Coats could face questions on the report Tuesday when he is scheduled to testify before the Senate Armed Services Committee.

Flynn's attorneys told the Senate intelligence committee on Monday that he will not turn over personal documents sought under the congressional subpoena, citing an "escalating public frenzy" against him. They also said the Justice Department's appointment of a special counsel has created a legally dangerous environment for him to cooperate with the Senate panel's investigation.

Hours later, Rep. Elijah Cummings, senior Democrat on the House oversight committee, said government documents he's reviewed showed inconsistencies in Flynn's disclosures to U.S. investigators in early 2016 during his security clearance review.

Cummings said Flynn appeared to have misled authorities about the source of a $33,000 payment from Russia's state-sponsored television network, failed to identify foreign officials with whom he met — including Russia's President Vladimir Putin — and glossed over his firing as chief of the Defense Intelligence Agency during the Obama administration. Cummings made his points in a letter asking the committee's chairman, Jason Chaffetz of Utah, to subpoena the White House for documents related to Flynn.

It's unclear from Cummings' letter whether Flynn would face legal jeopardy for his answers to security clearance investigators. But in an April statement Cummings warned that falsifying or concealing material facts on security clearance reviews are federal crimes and convictions could lead to fines and up to five years imprisonment.

Flynn attorney, Robert Kelner, declined to comment on Cummings' assertions.

Trump appointed Flynn, a retired U.S. Army lieutenant general and top military intelligence chief, as his top national security aide in January, only to fire him less than a month later. The White House has said that Flynn had misled top U.S. officials, including Vice President Mike Pence, about his contacts with Russian officials, including Russia's ambassador to the U.S.

Cummings and other Democrats have blasted Trump and his team for failing to more carefully check Flynn's background before they brought him to the White House, while the Trump administration has attempted to blame the Obama administration for failing to properly vet Flynn earlier.

Cummings said Monday that Flynn provided inconsistent or misleading statements to U.S. security clearance investigators in early 2016 during the renewal of his credentials.

Cummings cited a government report in March 2016 that he said showed the retired Army general telling authorities that payments he received for his 2015 trip to Moscow were paid by "U.S. companies." In fact, the oversight committee released detailed email and payment records months ago showing that the source of Flynn's payment of more than $33,000 was RT, the Russian state-sponsored television network that has been labeled a propaganda network by U.S. intelligence.

The payments, which were made through Flynn's U.S.-based speakers bureau, stemmed from Flynn's trip to Moscow to appear at an RT gala, where he sat at the head table with Putin.

In his letter, Cummings also cited a standard security clearance question that asks respondents to disclose contacts with foreign governments or their representatives. According to Cummings, Flynn told investigators he had only "insubstantial contact" with foreign nationals over the previous seven years and he did not detail the names of any foreign officials he had met. Among those omitted were Putin, RT officials and Russian military intelligence officials whom Flynn had met in Moscow in 2013 as part of his duties as defense intelligence chief.

Cummings said he found it difficult to understand how Flynn could have characterized his dinner with Putin as "insubstantial contact."

"General Flynn had a duty to be truthful in his security clearance renewal form and during his interview with security clearance investigators," Cummings wrote, noting that he's been in contact with the Justice Department and the newly appointed special counsel about his findings.

Meanwhile, the Senate committee's subpoena to Flynn focused on his interactions with Russian officials. It sought a wide range of information and documents about his and the Trump campaign's contacts with Russians dating back to June 2015.

Flynn's response stressed that his decision to invoke his constitutional protection was not an admission of wrongdoing but rather a response to a political climate in which Democratic members of Congress are calling for his prosecution. The attorneys also said that if Flynn complied with the committee's request, he could be confirming the existence of documents, an act that itself could be used against him.

Trump has defended Flynn since his ouster and called on him to strike an immunity deal because Flynn was facing a "witch hunt." The president's comments were in stark contrast to his harsh words during the 2016 campaign for people who received immunity or invoked the Fifth Amendment in the probe of Hillary Clinton's use of a private email server.

"If you're innocent, why are you taking the Fifth Amendment?" Trump asked in a campaign rally in Iowa in September.

Trump himself walked back into the Russia controversy during his visit to Israel, volunteering that he "never mentioned the word or the name Israel" during his recent Oval Office conversation with top Russian diplomats.

That comment referred to revelations that he divulged classified information about an Islamic State threat in his May 10 meeting in the Oval Office with Russia's foreign minister and ambassador. U.S. officials have said the information originated with Israel. However, it has not been alleged that Trump told the Russians that Israel was the source.

Flynn's decision does not fully close the door on future cooperation with the committee. But if congressional committees move to grant Flynn immunity, they would probably have to enter into discussions with the special counsel, Robert Mueller, to determine whether that could impede the FBI's case.