WASHINGTON — Legal experts say the former FBI director’s memo relating President Donald Trump’s request to end an investigation of his national security adviser could be used to build an obstruction of justice case against the president.

But they say criminal charges of interfering with an investigation are difficult in ordinary circumstances. It’s also an open question whether a sitting president can even face criminal charges, or whether attempts to hold him accountable for wrongdoing can only proceed through impeachment.

The White House has denied the account in the memo written by fired FBI Director James Comey of a conversation he had with Trump in February. Still, to a prosecutor building an obstruction of justice case, the facts may be compelling. They were revealed in recent days by Comey associates.