Donald Trump. REUTERS/Kevin Lamarque

President Donald Trump pardoned America's "toughest sheriff," Joe Arpaio, on Friday.

He reportedly asked Attorney General Jeff Sessions in the spring about dropping the federal investigation into Arpaio.

This development could serve as a critical data point in the FBI's Russia investigation, which reportedly includes whether Trump obstructed justice.

The Washington Post's report on Saturday that President Donald Trump asked Attorney General Jeff Sessions about dropping the case against former Arizona Sheriff Joe Arpaio offers a window into Trump's thinking.

In particular, Trump's conversation with Sessions and subsequent decision to pardon Arpaio could shed new light on Trump's motives in speaking to James Comey, then the FBI director, one day after former national security adviser Michael Flynn resigned about ending the investigation into Flynn.

Arpaio, an early and enthusiastic Trump surrogate, was convicted of criminal contempt in July for violating a court order to stop racially profiling Latinos.

When Trump asked Sessions in the spring whether it would be possible to drop the federal criminal investigation into Arpaio, Sessions told Trump such a move would be inappropriate but that Trump could pardon Arpaio if he were convicted, The Post reported, citing three people familiar with the conversation.

Trump granted the pardon on Friday evening, sparking fierce backlash from liberals and some conservatives.

But some legal analysts also said the pardon and Trump's previous actions may serve as an important piece of evidence to Robert Mueller, the special counsel leading the FBI's investigation into Russia's interference in the 2016 presidential election. That investigation has reportedly expanded to include whether the president attempted to obstruct justice.

Specifically, Trump's decision to pardon Arpaio is key to determining his intent in speaking with Comey in February about dropping the Flynn investigation and then firing Comey in May after he refused to do so, said Renato Mariotti, a former federal prosecutor.

How Comey fits in

James Comey, the former FBI director. REUTERS/Jonathan Ernst According to Comey's testimony before the Senate Intelligence Committee in June, Trump privately told Comey, who was spearheading the FBI's Russia probe at the time, "I hope you can let this go," referring to the Flynn investigation.

Several Trump allies and Republican lawmakers have since said that those words do not prove Trump tried to obstruct justice by asking Comey to drop the investigation and subsequently firing him.

Sen. Jim Risch of Idaho, for instance, zeroed in on the statement during Comey's testimony.

After reading out loud Comey's recollection of Trump's statement, Risch said, "He did not direct you to let it go."

"Not in his words, no," Comey replied.

"He did not order you to let it go," Risch said.

He later asked Comey whether he knew "of any case where a person has been charged for obstruction of justice or, for that matter, any other criminal offense, where this — they said, or thought, they hoped for an outcome?"

When Comey said he took Trump's words as a direction from the president of the United States, Risch said, "You may have taken it as a direction, but that's not what he said."

The main thing that Mueller — who was put in charge of the Russia investigation after Trump fired Comey — would need to prove in an obstruction-of-justice case is whether Trump acted with corrupt, or unlawful, intent when he asked the FBI director to drop the Flynn investigation.

"[Trump's] defense would be that he thought it was appropriate to end the Flynn investigation because it was meritless and that there was nothing wrong with him, as president, making that determination," Mariotti told Business Insider.

But the president's conversation with Sessions and decision to pardon Arpaio demonstrates that "this has become a pattern of activity where he tries to end investigations of his friends," he said. "Everything he said, did, and was told as to Arpaio is relevant to help us understand what he was thinking when he tried to end the Flynn investigation."

Moreover, despite statements from Trump's allies and administration officials who painted Trump's comments to Comey as musings and not as a direct order, Arpaio's pardon suggests "that he was serious about ending investigations as to his friends" and that it wasn't "just idle talk."

A pattern emerges

Trump and Michael Flynn. George Frey/Getty Images

Experts say that intent in obstruction-of-justice cases is difficult to prove.

Obstruction of justice is broadly defined — it involves any conduct in which a person willfully interferes with the administration of justice.

Trump's statements don't individually rise to the level of obstruction of justice, said Jens David Ohlin, an associate dean at Cornell Law School who's an expert on criminal law.

"The obstruction of justice would flow from the entire landscape of Trump's behavior: telling Comey to back off on the Flynn investigation, firing him when he wouldn't, and then admitting on national television that he dismissed Comey because of the Russia investigation," Ohlin told Business Insider.

He said that Trump's telling Russian officials in the Oval Office that Comey was "a real nut job" and that firing him had taken "great pressure" off of him added weight to the inference that Trump used his executive authority to fire Comey to stymie a federal investigation.

The FBI's investigation has progressed in recent weeks. Mueller's team reportedly issued subpoenas to several Washington lobbying firms that have worked with Flynn's and Trump campaign chairman Paul Manafort's consulting groups, and the special counsel is also scrutinizing a June 2016 meeting that Donald Trump Jr. had with a Kremlin-connected lawyer and a former Soviet military intelligence officer.

The Post additionally reported earlier this month that the FBI conducted a predawn raid on Manafort's home in July and left "with various records."

And recent revelations that lower-level aides were in touch with Russian contacts during the campaign are likely to generate more leads, as well as indicate that people who were not previously at the center of the controversy will be of interest to the congressional intelligence committees and the special counsel.