As Robert Mueller III probed whether President Donald Trump attempted to obstruct justice while in office, one person emerged as a key witness in the special counsel’s inquiry: former White House counsel Donald McGahn.

The special counsel’s nearly 450-page redacted report was released Thursday, relaying the findings of Mueller’s nearly two year inquiry into potential links between the Trump campaign and the Kremlin in the 2016 election, and whether Trump obstructed justice while in office.