Several federal statutes criminalise actions that impede official investigations.
While some examples of illegal ways to thwart the justice system are specific - such as killing a witness or destroying evidence - the law also includes broad, catch-all prohibitions. For example, Sections 1503, 1505 and 1512 of Title 18 have variants of language making it a crime if someone corruptly "obstructs, influences or impedes any official proceeding".
Could these cover asking the Federal Bureau of Investigation (FBI) director to drop part of an investigation, and later firing him?
In theory, yes. Such statutes were broadly drafted.
Former federal prosecutor Barak Cohen said the theory would be that President Donald Trump pressured Mr James Comey to stand down on Mr Michael Flynn because he feared the investigation could affect him personally.
Last week, Mr Trump removed Mr Comey from his post as FBI director and claimed that Mr Comey had told him three times he was not under investigation.
"While I greatly appreciate you informing me, on three separate occasions, that I am not under investigation, I nevertheless concur with the judgment of the Department of Justice that you are not able to effectively lead the bureau," he wrote to Mr Comey.
Mr Cohen said that itself could be a piece of circumstantial evidence that Mr Trump was trying to impede an ongoing probe.
Mr Trump also said in an NBC News interview that the Russia probe was on his mind when he fired his FBI director.
Mr Cohen said Mr Comey's memo was "direct evidence" against Mr Trump, but he noted that it came out only after the termination.
"The question is, if the President committed the crime or attempted to commit the crime of obstruction, and you serve the Constitution and not the President, why didn't you say something about this before you were fired?" Mr Cohen said.
NEW YORK TIMES, WASHINGTON POST