Probing the president for possible crimes is a complicated affair, even if convincing evidence of a crime is found. Reporter Tony Mauro noted the addition of Dreeben may signal that "Mueller may be seeking advice on complex areas of criminal law, including what constitutes obstruction of justice". At his hearing last week, Comey testified that he believed he was sacked on May 9 "because of the Russian Federation investigation".
Comey's statement before the House Intelligence Committee upset Trump, who has repeatedly denied that any coordination with the Russians took place.
Mueller is interviewing as early as this week three top intelligence officials as part of the probe, the Post reported, citing "five people briefed on the requests". He also said Trump had privately urged him to pull back on the investigation into former national security adviser Michael Flynn - a claim that the president has denied. Coats later told lawmakers that he never felt pressured to intervene.
Coats and Rogers refused to comply with the president's requests, officials said.
The president and his surrogates are already on the warpath, falsely suggesting that Comey engaged in improper or illegal conduct in leaking his memos and, putting out the line via Kellyanne Conway, that members of Mueller's legal team had given to Democratic politicians (and therefore were biased).
As part of the probe, the special counsel has also gathered Comey's written accounts of his conversations with Trump. Investigators are also considering the conversations Comey and the president had leading up to that point.
As former Sen. John Danforth, who as a special counsel investigated the 1993 Waco siege disaster, has said: "You can't have an investigation that is dribbled out there in bits and pieces and suspicions".
Comey also testified that Trump asked him to publicly deny that the president was being investigated by the Federal Bureau of Investigation.
Robert Mueller, a former FBI director, is leading an investigation into alleged Russian meddling in the 2016 U.S. presidential election and possible collusion with the Trump campaign.
He declined to say whether he thought Trump sought to obstruct justice, saying that would be up to Mueller to sort out.
The emergence of the obstruction of justice inquiry may make it harder for Trump to have Mueller removed.
The new allegations against Trump center on his own admission that he fired Comey because of the Russian Federation investigation, and suggestions he asked several top intelligence officials for their help altering the inquiry's direction. And I don't care if you're Republican, Democrat - it doesn't matter.
The president vehemently denies any collusion between himself or any of his associates and Russian Federation.
The president did not directly address allegations that he is being probed for possibly obstructing justice - a potentially impeachable offense. Two days later, Comey is questioned by a Republican-led House Committee about his recommendation.