WASHINGTON (Reuters) - A key element of Special Counsel Robert Mueller’s inquiry into Russia’s role in the 2016 U.S. election was whether President Donald Trump unlawfully acted to impede the investigation, a crime known as obstruction of justice.

According to U.S. Attorney General William Barr, Mueller’s nearly 400-page report on his findings presents evidence on both sides of the question, and while it “does not conclude that the president committed a crime, it also does not exonerate him.”

But Barr, two days after Mueller submitted the confidential report on March 22, told U.S. lawmakers in a four-page letter that he as attorney general concluded that the evidence amassed by the special counsel “is not sufficient” to establish that Trump committed criminal obstruction of justice.

Federal law defines obstruction of justice as actions that “corruptly or by threats or force, or by any threatening letter or communication, influences, obstructs, or impedes, or endeavors to influence, obstruct, or impede, the due administration of justice.”

The public may soon get a chance to make its own conclusions. Barr on Tuesday said in congressional testimony he plans to release the report within a week, with portions blacked out to protect certain categories of sensitive information.

Mueller’s investigation into the Trump campaign’s numerous contacts with Russia and whether he committed obstruction of justice has cast a cloud over his presidency heading into his 2020 re-election bid.

Here is an explanation of key events relating to whether Trump committed obstruction of justice.

OVAL OFFICE MEETING WITH COMEY

Then-FBI Director James Comey and other U.S. intelligence officials attended a Valentine’s Day counterterrorism briefing at the White House on Feb. 14, 2017. After the briefing, Trump, who had taken office just weeks before on Jan. 20, asked everyone but Comey to leave the room, according to testimony Comey gave to Congress in June 2017.

The White House is seen after Special Counsel Robert Mueller handed in his report to Attorney General William Barr on his investigation into Russia's role in the 2016 presidential election and any potential wrongdoing by U.S. President Donald Trump in Washington, U.S., March 22, 2019. REUTERS/Leah Millis

According to Comey, Trump said he wanted to talk about Michael Flynn, who had resigned under pressure a day earlier amid revelations about his contacts with Russia’s ambassador to the United States, Sergei Kislyak, while Trump’s predecessor Barack Obama was still president.

“I hope you can see your way clear to letting this go, to letting Flynn go,” Trump told Comey, according to memos Comey wrote about the conversation.

Comey testified that he interpreted that as Trump giving him a “direction” regarding an FBI investigation into Flynn’s false statements about his Russian contacts: to drop the inquiry. It was one of handful of private conversations between Comey and Trump in early 2017. Trump repeatedly said he expected loyalty, Comey said. Comey said he viewed the conversations as unusual given the FBI’s long history of independence from the White House.

In March 2017, Trump held a private meeting with Director of National Intelligence Dan Coats and then-CIA Director Mike Pompeo, now secretary of state, and asked them to intervene and get the FBI to back off its Flynn investigation. Despite these conversations, the FBI continued to investigate the Trump campaign’s contacts with Russia during the 2016 election.

TRUMP FIRES COMEY

Trump fired Comey on May 9, 2017. The public explanation given by the White House was that Comey had mishandled a 2016 investigation into the use of a private email account and server by Hillary Clinton, Trump’s 2016 Democratic opponent. Then-Attorney General Jeff Sessions and Justice Department No. 2 official Rod Rosenstein signed letters recommending Comey’s firing on those grounds.

The next day, Trump had a private White House meeting with visiting Russian officials. “I just fired the head of the FBI. He was crazy, a real nut job,” Trump said, according to reporting by the New York Times. “I faced great pressure because of Russia. That’s taken off,” Trump added.

On May 11, 2017, Trump again appeared to tie Comey’s firing to the Russia investigation in an interview with Lester Holt of NBC News. Trump told Holt he was going to fire Comey regardless of Rosenstein’s recommendation. “And in fact when I decided to just do it, I said to myself, I said, ‘You know, this Russia thing with Trump and Russia is a made-up story,’” the president said.

The weekend before firing Comey, Trump dictated to an aide a meandering four-page letter explaining his reasons for firing Comey, which was never sent, according to the New York Times. That letter, which Mueller has obtained, stated Trump’s displeasure with Comey’s handling of the Russia investigation, the Times reported.

TRUMP ORDERS MUELLER’S FIRING

Rosenstein appointed Mueller as special counsel to take over the Russia investigation after Trump fired Comey. In June 2017, news reports surfaced that Mueller was also investigating possible obstruction by Trump.

Trump ordered White House counsel Don McGahn to fire Mueller, citing alleged conflicts of interest, but McGahn refused and threatened to quit, the New York Times reported in January 2018. McGahn was concerned that firing Mueller would fuel accusations that the White House was trying to obstruct the investigation, and McGahn’s refusal prompted Trump to back off the order, the Times reported.

STATEMENT ON TRUMP TOWER MEETING

Trump’s eldest son, Donald Trump Jr., set up a June 2016 meeting at Trump Tower in New York with a Kremlin-linked lawyer, Natalia Veselnitskaya, and other Russians who had offered the campaign damaging information on Clinton. After being promised “dirt” on Clinton, Trump Jr. wrote in an email, “I love it.”

When news of the meeting broke in July 2017, Trump Jr. issued a misleading statement saying the meeting was set up to discuss adoption policy, not politics, before later admitting he had been expecting intelligence on Clinton. At issue is his father’s role in drafting the statement. White House advisers later acknowledged that the president dictated the statement put out in his son’s name, after initially denying his involvement. Misleading journalists and the public is not a crime, but the shifting explanations could be seen as evidence of Trump’s intent to impede the investigation, legal experts said.

PRESIDENTIAL PARDONS

Trump lawyer John Dowd in 2017 repeatedly broached the idea of Trump giving presidential pardons to former advisers charged by Mueller, the New York Times reported in March 2018. Some legal experts have said that dangling a pardon in front of witnesses in hopes of influencing their testimony could constitute obstruction of justice.

TRUMP ASSAILS SESSIONS OVER RECUSAL

Normally, the U.S. attorney general would have overseen the Russia inquiry. But Sessions in March 2017 recused himself because of his own contacts with Kislyak, the Russian ambassador, while serving as a Trump campaign adviser. Oversight fell to Rosenstein, who subsequently appointed Mueller.

The New York Times reported that Trump had pressured Sessions not to recuse himself and erupted in anger when the attorney general did so. Sessions wrote a resignation letter, but Trump rejected it on the advice of advisers, according to news reports.

In July 2017, Trump on Twitter called Sessions “beleaguered” and accused him of taking “a VERY weak position on Hillary Clinton crimes.” In June 2018, Trump tweeted: “The Russian Witch Hunt Hoax continues, all because Jeff Sessions didn’t tell me he was going to recuse himself.” In September 2018, Trump told an interviewer, “I don’t have an attorney general. It’s very sad.” Trump ousted Sessions in November 2018.

Trump’s hectoring of Sessions could be cited by Mueller as evidence of an intent to obstruct the probe, legal experts said.