 -- Relatives of retired Lt. Gen. Michael Flynn, one of the key subjects of multiple investigations into Russian interference in the 2016 presidential election, made a rare public statement Monday to call attention to the “tremendous financial burden” the probes are placing on former advisers to President Donald Trump.

“The enormous expense of attorneys’ fees and other related expenses far exceed their ability to pay,” said Michael Flynn’s siblings Joe Flynn and Barbara Redgate.

They made the statement to bring attention to a legal defense fund they have formed to help raise money for their brother, who briefly served as Trump’s first national security adviser, after joining the Trump campaign as an adviser in 2016. Michael Flynn was forced to resign after just 24 days as national security adviser, after it was revealed that he misled Vice President Mike Pence about his conversations with Russian officials during the presidential transition.

Flynn, a decorated military officer who served as the director of the Defense Intelligence Agency from 2012 until his retirement in 2014, was out of the spotlight only briefly. He emerged as an early target of special counsel Robert Mueller, who was appointed to investigate Russian interference in the election. Congress also began pursuing information from Flynn, but he has so far provided only limited information to the House and Senate committees charged with investigating election interference.

Rep. Elijah Cummings told ABC News last week that his committee was looking deeply into Flynn’s foreign contacts in 2015 and 2016, which Cummings said Flynn failed to disclosed during reviews of his security clearance. Those communications — which involved high-ranking foreign officials and business executives — were so numerous that they could not have been inadvertent omissions or incidental contacts, Cummings said.

“He has, over and over again, omitted information that he should have disclosed,” Cummings said. “It’s not an aberration, and that’s clear.

The omissions are a serious matter. While Cummings said intentionally omitting foreign contacts when applying for security clearance can carry a five-year prison term, he acknowledged that penalties are rarely that severe. The leverage that those alleged transgressions provide, however, could prove useful to prosecutors seeking to use the threat of prosecution to compel Flynn’s assistance in the broader investigations into Russian interference in the election.

Former FBI Director James Comey provided a window into that strategy during his three hours of testimony before the Senate Intelligence Committee this year.

“There is always a possibility if you have a criminal case against someone and you bring them in, squeeze them, flip them, [that] they give you information about something else,” Comey said.

Flynn has remained silent publicly about the ongoing probes. ABC News caught up with him this summer at a beach in Newport, Rhode Island, his hometown. “I’m just having a great time with the family here,” he said. “I’m doing good. I’m not going to make any comments.”

His family established the defense fund, his relatives said, “to help ensure that he can defend himself," and this morning he tweeted from his personal account for the first time since late last year to express his gratitude.

Referring to his wife, Lori Flynn, Michael Flynn wrote in two tweets, "Lori and I are very grateful to my brother Joe and sister Barbara for creating a fund to help pay my legal defense costs. We deeply appreciate the support of family and friends across this nation who have touched our lives."

Experts told ABC News that such funds are complicated entities, in part because efforts must be made to ensure that they do not become vehicles for outside parties to attempt to influence his willingness to cooperate.

Stephen Gillers, an ethics expert who holds the Elihu Root Chair at New York University School of Law, says defending a felony prosecution can cost as much as $10 million, and he is adamant about a defendant’s right to raise money to offset those expenses.

“It is none of the government’s business about how a defendant funds his case,” Gillers told ABC News. “The only time the government has an interest is if the money is tainted money, the fruits of your crime, [but] I do not think there is an issue of tainted money in the Flynn prosecution.”

His legal defense fund will not accept contributions from foreign nationals, anonymous sources, the Trump campaign or the Trump Organization, according to a source close to Flynn.

“Mike devoted 33 years of his life to our country serving in the United States Army, spending years away from his family while he fought this nation’s battles overseas, including the war on terror,” his brother and sister said as part of their plea for financial support.