What is obstruction of justice?
It's against the law to frustrate or try to frustrate an investigation, even if no underlying crime was committed. Lying to federal investigators is a crime on its own and so is acting more broadly to prevent or delay or otherwise interfere with the course of their work.
It's also politically important: The last two occasions on which Congress has filed articles of impeachment against sitting presidents — against Richard Nixon and later, Bill Clinton — they included allegations about obstruction.
But it's a complicated case for a prosecutor to make because the law requires proving that someone had a corrupt intent to obstruct justice and then acted in service of that aim. That means getting inside a subject's head and convincing a jury about what happened, which is difficult.
Why do critics accuse President Trump of obstructing justice?
One theory of the case is that Trump may have broken the law by asking FBI Director James Comey not to pursue the case of former national security adviser Mike Flynn.
Flynn lost his position in the White House after lying to Vice President Mike Pence about a conversation Flynn had with Russia's ambassador before Trump was inaugurated, although the specific details from that time aren't fully known. What Comey says is that Trump cleared the room on Feb. 14, 2017, and, with just the two of them present, asked him to "let this go."
Trump denies that. He also denies an earlier conversation described by Comey in which Comey says the president asked him for his "loyalty." What is not in dispute is that Trump fired Comey on May 9, 2017.
Did the president fire Comey to try to intimidate the FBI or delay its investigation? What other actions might have taken before the fact? Did he ask other people in the administration, whether Attorney General Jeff Sessions to Director of National Intelligence Dan Coats or others, to try to get the FBI to ease up on Flynn?
This could be a major question for Justice Department special counsel Robert Mueller.
What does Trump say?
Trump has connected his firing of Comey to the Russia investigation in some public comments, but the president and other administration officials also have given other reasons for the firing. The formulation of Trump's decisions, their timing and other matters are likely a subject of close focus by Mueller, who was appointed to take over the Russia investigation after Comey's firing.
The president denies any suggestion his campaign conspired with the Russian attack on the 2016 election and calls the subsequent investigation a "witch hunt." On obstruction, he also denies wrongdoing, but the matter is slightly less clear cut because he is willing to "fight back," as Trump has described on Twitter.
What does Comey say?
Comey made clear from the first that he believes a president has the power to fire an FBI director at any time. Comey talked to NPR's Steve Inskeep and Carrie Johnson on May 16, 2018. He was asked about obstruction and said that although Trump could be guilty, he didn't have a full picture of the evidence that investigators may be assembling.
"I can't see all of it," he said. "And it could be obstruction of justice. It would depend upon what the full scope of the evidence is with respect to intent, because obstruction of justice requires a demonstration of corrupt intent. And I'm just a witness when it comes to that particular incident, the February 14th incident. And so I don't know what the evidence is that the prosecutor and investigators have gathered with respect to intent. It maybe there, it may not be. I just — it doesn't make sense, and I can't responsibly offer an opinion from my vantage point."