Top officials within the FBI were told by then-Acting Director Andrew McCabe that they should consider themselves as potential witnesses if the investigation by special counsel Robert Mueller looks at potential obstruction of justice by President Trump.

The message from McCabe came soon after Mueller's appointment last May, according to a new report by Vox.

The report cited two "senior federal law enforcement officials," and quoted one of them as saying:

"What you are going to have is the potential for a powerful obstruction case," a senior law enforcement official said. "You are going to have the [former] FBI director testify, and then the acting director, the chief of staff to the FBI director, the FBI's general counsel, and then others, one right after another. This has never been the word of Trump against what [James Comey] has had to say. This is more like the Federal Bureau of Investigation versus Donald Trump."

Theories for how an obstruction of justice case would proceed largely rely on the tying together President Trump's request of then-FBI Director Comey to drop an investigation into Trump's former national security advisor Mike Flynn, Comey's refusal to do so, and then the eventual firing of Comey by the president.

In May, McCabe told the Senate Intelligence Committee, "There has been no effort to impede our investigation to date."

The Vox report said Comey, aside from keeping memos of his conversations with President Trump, also had conversations with top FBI officials about the talks, usually within 24 to 48 hours after they occurred.

The pressure for an obstruction case has been less talked about in recent weeks after Donald Trump Jr. released a string of emails that show he, Jared Kushner, and Trump campaign manager Paul Manafort met with a Russian lawyer in June of last year in an attempt to obtain damaging information on the Clinton team. After those revelations, there's been increased speculation that the Mueller team would focus on collusion instead of obstruction of justice.

Defenders of the president argue that the meeting with Trump Jr. and others wasn't collusion, and that no other proof of collusion has been shown despite numerous ongoing investigations and a torrent of leaks from congressional committees and from the intelligence agencies that have harmed the administration.