EXPLAINER -Why an obstruction case against Trump was so difficult

By Jan Wolfe

WASHINGTON, March 24 (Reuters) - U.S. Attorney General William Barr's conclusion that President Donald Trump did not obstruct justice, revealed publicly on Sunday in a letter to lawmakers, reflects inherent difficulties in proving such an obstruction case, legal experts said.

Barr sent a four-page letter to congressional leaders summarizing a report by U.S. Special Counsel Robert Mueller handed in on Friday, which remains confidential.

Mueller was appointed to investigate whether Trump's campaign conspired with Russia to influence the 2016 election and to probe whether the president later unlawfully sought to obstruct the probe.

Mueller vindicated Trump on the core question of Russian collusion, writing in his report that "the investigation did not establish that members of the Trump campaign conspired or coordinated with the Russian government in its election interference activities," according to Barr's letter.

While Mueller stopped short of determining whether Trump obstructed justice, Barr concluded there was insufficient evidence to bring such a case.

Following are some reasons why proving an obstruction case against Trump was so difficult and what may lay ahead.

WHAT DID MUELLER SAY ABOUT AN OBSTRUCTION CASE AGAINST TRUMP?

Mueller said he did not reach a finding on whether Trump obstructed justice and set out "evidence on both sides of the question," according to Barr's letter.

Barr did not make this evidence public, noting only that Mueller said there were "difficult" factual and legal questions raised by an obstruction case.

Muller said "while this report does not conclude that the President committed a crime, it also does not exonerate him," according to Barr's letter.

Barr, however, found there was insufficient evidence to establish an obstruction case against Trump.

WHY WAS IT DIFFICULT TO PROVE OBSTRUCTION AGAINST TRUMP?

A federal law makes it a crime to attempt "to influence, obstruct or impede the due administration of justice."

To prove obstruction, prosecutors must show an individual acted with a "corrupt," or improper motive - a specific intent to impede an investigation.

Mueller's obstruction investigation likely focused on Trump's interactions with former FBI director James Comey, legal experts said.

According to Comey, in February 2017 Trump asked him to back off an investigation into national security adviser Michael Flynn over Flynn's contacts with Russia. Trump eventually fired Comey in May 2017.

"I hope you can see your way clear to letting this go, to letting Flynn go ... He is a good guy," Trump told Comey, according to a memo written by the former FBI director.