(CNN) The President's eldest son (Donald Trump Jr.) son-in-law (Jared Kushner) and former campaign chairman (Paul Manafort) are sitting down with the staff and members of several Senate committees, but sources tell CNN some of the meetings will be behind closed doors and not under oath.
But even if the men don't raise their right hand before speaking, they still have to tell the truth.
Federal law makes it a crime to "knowingly and willfully" give "materially" false statements to Congress, even if unsworn -- which is not to be confused with the more general crime of perjury for lying under oath.
The consequences for either crime are serious: one can face up to five years in prison.
Like many criminal statutes, however, proving a witness "knowingly" sought to mislead sets a high bar for prosecution -- meaning the omission can't be merely a mistake or accident. And the "materiality" requirement means the false statement has to actually matter -- i.e., a tendency to influence the listener.
While it is rare to see charges filed for lying to Congress, there is precedent.
John M. Poindexter, President Ronald Reagan's national security adviser, was sentenced to prison for lying to Congress about key details of the Iran-Contra affair.
So what's the point of making a witness take the oath if lying to Congress is still a crime?
"There's something ceremonial about having someone under oath," said Renato Mariotti, a former federal prosecutor who is now a partner at Thompson Coburn LLP.
"One can argue that the difference is theatrical as much as legal," agrees University of Chicago Law Professor Daniel Hemel, which is "not to say the oath is useless" as it may have a "psychological impact on the speaker."
CLARIFICATION: This post has been updated to explain the circumstances of Poindexter's sentencing. He was initially convicted and sentenced to prison but his felony convictions were later overturned.