Obstruction of justice cases often comes down to whether prosecutors can prove defendants' mental state when they committed the act, legal specialists said.
It is not enough to show that a defendant knew the act would have a side consequence of impeding an investigation; achieving that obstruction must be the specific intention.
"You have to realise that - as with any other sort of criminal law - intent is key, and intent here can be difficult to prove," said Mr Barak Cohen, a former federal prosecutor who now does white-collar defence work at the Perkins Coie law firm.
The laws governing obstruction of justice require prosecutors to show a person "corruptly" tried to influence a probe - meaning investigators have to find some evidence of what a person was thinking when taking a particular action.
In this case, analysts said, that would mean analysing the details of US President Donald Trump and then FBI director James Comey's conversation, assessing what else was happening at the time, and possibly talking to Trump associates who had talked with the President about what he wanted to do.
"It depends on what he said and how he said it," said Mr Edward MacMahon Jr, another criminal defence lawyer. He noted there are circumstances - such as the Central Intelligence Agency worrying about disclosure of classified information - in which one component of the executive branch discourages prosecutors from pursuing a case.
The Justice Department's Office of Legal Counsel has opined in the past that the President cannot be indicted or prosecuted at all, "because it would impermissibly interfere with the President's ability to carry out his constitutionally assigned functions and thus would be inconsistent with the constitutional structure".
Mr Trump could be impeached, though, for "treason, bribery, or other high crimes and misdemeanours". In the articles of impeachment against president Richard Nixon in the Watergate scandal - which were never taken up by the full House of Representatives - legislators cited obstruction of justice.
NYTIMES, WASHINGTON POST