US 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 US Special Counsel Robert Mueller handed in on Friday, which remains confidential.

Also read:Mueller finds no Trump-Russia conspiracy

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 Muller say?

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 is it difficult to prove obstruction?

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.

Some legal experts have said in the last year that Trump's intent was clearly to block an investigation, and that there was a strong obstruction of justice case to be made. But others have said that establishing that Trump acted with the “corrupt” intent of blocking the investigation would have been difficult. Trump could have argued he was simply vouching for Flynn's character, and not pressuring Comey to drop an investigation, these lawyers said.

In an interview with NBC Nightly News anchor Lester Holt that aired two days after the firing, Trump said he was thinking of “this Russia thing” when he fired him. But Trump also said in the NBC interview and in others that he fired Comey because he was incompetent, weakening a potential obstruction case, experts said.

Why the clean chit?

An obstruction of justice case against Trump was also undermined by Mueller's findings that the Trump campaign did not conspire with Russians to interfere in the election.

Obstruction of justice is usually coupled with some underlying wrongful act that is being covered up, experts said. Barr cited this shortcoming in his letter, saying that, “while not determinative, the absence of such evidence bears upon the President's intent with respect to obstruction.”

Barr's letter also notes that obstruction, like any crime, requires to be proven beyond a reasonable doubt in court. He also noted that many of the actions described in Mueller's report “took place in public view”. Obstruction of justice typically describe secret efforts to impede a court proceeding, and Barr may have viewed the public nature of Trump's acts as weakening the case against him.

Could Congress pick up the obstruction charges?

Yes. Democrats took control of the House of Representatives in January, giving them investigatory powers including the ability to issue subpoenas.

US House of Representatives Speaker Nancy Pelosi and Senate Democratic leader Chuck Schumer said in a joint statement on Sunday that the fact that Mueller had not cleared Trump of obstruction “demonstrates how urgent it is that the full report and underlying documentation be made public without any further delay.”

Impeachment in the future?

The US Constitution empowers Congress to remove a President from office for “high crimes and misdemeanors”, including obstruction of justice, through a process of impeachment.

In an impeachment proceeding, lawmakers have broad leeway to define obstruction of justice and are not bound by Barr's determination, legal experts said. But impeachment is a very political process, and appears unlikely because Trump's Republican party controls the Senate. Some Democratic lawmakers had hoped Mueller's investigation would lay a groundwork for impeachment, but that hope has now largely faded.