(Reuters) - By saying he believed Donald Trump directed him to end a probe into a former national security adviser, former FBI Director James Comey added fuel on Thursday to critics’ accusations that the U.S. president engaged in obstruction of justice.

Former FBI Director James Comey testifies before a Senate Intelligence Committee hearing on Russia's alleged interference in the 2016 U.S. presidential election on Capitol Hill in Washington, U.S., June 8, 2017. REUTERS/Jonathan Ernst

Such an offense could be the basis for eventual impeachment proceedings against Trump. But proving obstruction of justice is far from clear-cut, because it hinges on intent, and legal experts differed sharply on whether it might apply to Trump and his dealings with Comey and the FBI probe.

Comey told the Senate Intelligence Committee he believed that Trump had directed him to drop the FBI’s probe into Michael Flynn, who was being investigated as part of a wider investigation into alleged Russian meddling in the 2016 presidential election and possible collusion by Trump’s campaign team. Comey also said he thought Trump fired him last month because he was unhappy with how he was conducting the Russia probe.

Marc Kasowitz, Trump’s personal lawyer, said after Comey’s testimony that “the president never, in form or substance, directed or suggested that Mr. Comey stop investigating anyone.”

The following describes obstruction of justice and whether Comey’s testimony could lead to either a prosecution or impeachment of the president.

What is obstruction of justice?

There are several state and federal laws that make it crime to interfere with a court case, law enforcement investigation or government administrative proceeding.

To convict someone of obstruction of justice, prosecutors need to show that person intended to interfere with an investigation or proceeding. It does not matter whether the person succeeded or not.

Can a president be charged with obstruction of justice?

The U.S. Constitution does not directly address whether the president can be criminally prosecuted. Many lawyers contend the president is therefore immune from prosecution. But this is an “unresolved legal debate,” said Andrew Wright, a professor at Savannah Law School.

The constitution does explicitly provide for impeachment of the president for “treason, bribery, or other high crimes and misdemeanors.” Congress is free to interpret high crimes and misdemeanors to include obstruction of justice, and it was one of the charges Congress prepared against President Richard Nixon over the Watergate scandal. Nixon resigned in 1974 before he could be impeached.

David Sklansky, a professor at Stanford Law School, said Congress can define obstruction of justice as it sees fit but would likely look to the federal criminal law for guidance.

However, impeachment is a political process that is currently viewed as highly unlikely given Trump’s fellow Republicans control both chambers of Congress.

The process begins in the House of Representatives, which votes to initiate impeachment proceedings much like a grand jury would vote to indict in a criminal case. A vote to impeach requires a simple majority and leads to a Senate trial in which a group of House lawmakers act as prosecutors and the senators serve as a jury. A two-thirds vote is required for conviction, which would result in the removal of the president from office.

Only two U.S. presidents have been impeached: Andrew Johnson in 1868 and Bill Clinton in 1998. Neither was convicted by the Senate.

Two House Democrats, Representatives Al Green and Brad Sherman, said this week they were drafting what are known as articles of impeachment against Trump. Their effort has gained little traction in Congress and even many Democratic members say the effort is premature.

Did Comey’s testimony support an obstruction charge?

Comey testified that Trump told him during a Feb. 14 conversation that Flynn was a “good guy” and that he “hoped” Comey could see his “way clear” to dropping the probe into him. Comey told the Senate panel he interpreted the president’s statement as a “direction.”

Several legal experts said the conversation could be construed as an act of obstruction. They said Trump’s intent to interfere with the investigation could be inferred from his conduct, such as telling advisors like Jared Kushner and Attorney General Jeff Sessions, Comey’s direct boss, to leave the room before the conversation with Comey.

“Asking others to leave the room could suggest the president was aware that there was something wrong with what he was doing,” Wright said.

What else might support an obstruction charge against Trump?

Laura Donohue, a professor at Georgetown University Law Center, said Trump’s firing of Comey could also constitute obstruction of justice. Though the president has the authority to fire an FBI director, it would be a crime to do so in order to halt the Russia investigation, she said.

Comey’s testimony suggested that was the president’s intent, said Donohue. “He wanted to scuttle the investigation – it is hard to see it any other way,” she said.

Wright also said Trump’s comment in a television interview soon after he fired Comey that he had the Russia issue in mind when he dismissed the FBI chief was evidence of his intent.

Comey sidestepped the question when asked on Thursday if he thought that Trump had engaged in obstruction of justice, saying this was a question for Robert Mueller, a former FBI chief appointed by the Justice Department last month as a special counsel to probe the Russia issue.

Why might Trump’s conduct fall short of obstruction?

Some legal experts said the president could say he was merely vouching for Flynn’s character and voicing concerns about how the Russia probe was interfering with his ability to function in office.

“Comey made it clear that the president made a request not a demand,” said Alan Dershowitz, a professor emeritus at Harvard Law School, “and (Comey) testified that he didn’t accede to it.”

Jonathan Turley, a professor at George Washington University Law School, also said there were no grounds for obstruction charge against Trump.

“This hearing may have hurt the president’s feelings but it doesn’t hurt his legal case,” he said. “I think the record just moves further away from any credible claim of obstruction of justice.”