Factbox: Did Comey's testimony support an obstruction of justice charge? 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

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.