Michael Flynn's chatty ways landed him under investigation for his ties with Russia. Now his silence and noncompliance with a Senate panel could land him in jail.

In theory.

In practice, there's little risk U.S. President Donald Trump's former national security adviser could be imprisoned for his refusal to hand over documents sought under subpoena by a Senate intelligence committee — even as two new subpoenas were served up this week in the latest escalation of the probe into Trump associates' alleged contacts with Russian officials.

'Luckily for Flynn, the Department of Justice is under the control of President Trump, who has already announced his support for Flynn.' - James Duane, Fifth Amendment expert and Regent University law professor

Flynn's invocation of the Fifth Amendment — a right to refuse to testify as a witness against himself — is so universally recognized, legal experts say, that the efforts to compel him to co-operate won't have much bite.

A contempt of Congress citation "means less than you would think," says former federal prosecutor Barak Cohen, with the Washington firm Perkins Coie.

The Senate would have to refer contempt charges to the U.S. attorney for the District of Columbia. Judicial proceedings could take years, potentially outlasting any public interest in the matter.

If a U.S. attorney did prosecute for failure to comply with a congressional subpoena, it would amount to a misdemeanour offence punishable by a fine of no more than $1,000 and up to 12 months in prison.

Jail time may be a scary prospect, but "typically, they will do nothing, especially when ... Fifth Amendment rights," are asserted, Cohen says.

Going to trial and getting convicted of contempt of Congress is also not an easy thing to do, says James Duane, a former federal criminal defence attorney and expert on the Fifth Amendment. It would fall to the Department of Justice to seek charges, Duane says.

"And luckily for Flynn, the Department of Justice is under the control of President Trump, who has already announced his support for Flynn," Duane says.

Even if Flynn were convicted, Trump could issue a presidential pardon.

No case in modern history

Although contempt of Congress citations have proceeded before, criminal law and constitutional scholars couldn't recall any case in modern history that led to imprisonment.

Flynn, who Trump fired in February after revelations he lied about talks with the Russian ambassador, is exercising caution. In March, a Senate intelligence committee rejected his bid to strike a deal for immunity from prosecution in exchange for testimony into alleged Russian meddling in the 2016 presidential election.

Although the Fifth Amendment broadly protects someone from being compelled to give self-incriminating testimony, a central question is whether that constitutional privilege also protects Flynn from refusing to produce documents.

"Congress has investigatory powers, and with that comes subpoena powers," says Senate expert Rachel Bovard. "My understanding is that the Fifth Amendment only protects someone from self-incrimination but doesn't protect that person from being compelled to produce existing documents."

Flynn's defiance of the subpoenas could be challenged by the Senate committee under the "foregone conclusion" doctrine. By this rationale, handing over the files would not be considered incriminating if the government already knows the documents exist."

Flynn's most likely legal argument is a reliance on the "act of production" doctrine for coverage under the Fifth Amendment — a rare situation in which the mere act of collecting or producing the requested evidence would be tantamount to admitting its existence or authenticity.

Trump National Security Advisor

Flynn invoked his constitutional right against self-incrimination in response to a subpoena from the Senate intelligence committee. (Carolyn Kaster/Associated Press)

"If I walked in and said, 'Here's the stuff you're looking for,' I'm just incriminating myself," Duane says. "Just by producing these damning documents, I'm giving evidence that can be used against me by forcing me to admit that I knowingly possessed these materials."

That would give the government the "benefit of an admission" they could use against the witness or suspect, Duane says.

'If you're innocent, why are you taking the Fifth?'

As Congress turns up the heat on Flynn, his past remarks disparaging use of the Fifth Amendment have come back to haunt him. Last June, Flynn ridiculed Bryan Pagliano, who worked for Hillary Clinton when she was at the State Department, for invoking the privilege during a deposition.

During the election campaign, Trump also questioned why some former Clinton staffers would invoke the Fifth Amendment.

"The mob takes the Fifth," Trump told an Iowa crowd. "If you're innocent, why are you taking the Fifth Amendment?

But pleading the Fifth should not be inferred as a legal admission of guilt, even if it is reputationally damaging, says Yvonne Tew, an associate professor of constitutional law at Georgetown University.

"You could be innocent and still assert the Fifth Amendment," she says. "However, when public figures invoke the Fifth, sometimes in the court of opinion people do infer guilt."