Special Counsel Robert Mueller is investigating whether Trump has been attempting to obstruct the federal investigation into Russian interference in the 2016 election. Obstruction of justice is a criminal act in which someone attempts to “influence, obstruct, or impede” the “due administration of justice.” Below is a brief overview of four incidents when Trump attempted to obstruct justice, explaining what happened in each case and why it is evidence of obstruction of justice.

1. Trump firing James Comey

What? In May 2017, Trump fired FBI Director James Comey, who was overseeing the investigation into whether members of the Trump campaign colluded with Russian nationals to interfere in the 2016 election. Shortly before he fired the director, Trump reportedly asked Comey to drop the investigation into his National Security Adviser Michael Flynn, which Trump denies. Following Comey’s firing, Trump even told a reporter that he had been thinking about the “Russia thing” when he made the decision to fire Comey.

Why? If Trump asked Comey to drop the investigation into a member of his administration, it is a clear example of obstruction of justice. Trump has offered multiple explanations for firing Comey, but firing Comey with the intention of undermining the probe constitutes obstruction of justice.

Watch: Trump obstructed justice by firing Comey

2. Trump’s role in creating misleading statement on the Trump Tower meeting

What? At Trump Tower in June 2016, Donald Trump Jr. met with Natalia Veselnitskaya, an attorney from Russia, who supposedly had “dirt” on Hillary Clinton. The meeting quickly went downhill when it became clear that the attorney did not have any incriminating evidence on Clinton. A report published by The Washington Post stated that Trump “personally dictated” a public statement about the meeting that misrepresented the meeting’s content as being “primarily” about an adoption program.

Why? When Trump helped to write the statement, he may have been attempting to mislead investigators about the purpose of Don. Jr’s meeting, obstructing the probe. Legal experts have said this incident is evidence of a pattern of Trump’s conduct and his intent to obstruct justice.

3. Trump leveraging pardon power to influence witnesses

What? Trump’s lawyer John Dowd reportedly told lawyers representing Paul Manafort and Michael Flynn that Trump might be willing to pardon them if they were charged in the Russia investigation.

Why? Legal scholars said this could be obstruction of justice because the pardon could incentivize witnesses into not cooperating with the investigation. Manafort and Flynn could refuse to cooperate with Mueller, knowing that they will be pardoned for any crimes they committed.

4. Trump pressuring Jeff Sessions over his recusal from the Russia investigation

What? Trump told White House counsel Don McGahn to convince Attorney General Jeff Sessions to not recuse himself from the Russia investigation, which he oversaw as attorney general. Sessions recused himself from the inquiry in March 2017. A week after Comey was fired, Deputy Attorney General Rod Rosenstein appointed Mueller as special counsel to head the investigation.

Why? Trump may have been trying to maintain control over the Russia investigation when he was asking Sessions to not recuse himself. Trump wanted Sessions to continue to oversee the probe, and not another Department of Justice official, so that he could influence the outcome.

Read: Trump has continued to obstruct justice