Who can testify against US President Donald Trump?

That is the question the American public is most concerned about right now, after the Special Advisor in charge of the Russia investigation established a grand jury to summon witnesses.

Vox news site quoted two senior federal law enforcement officials as saying that as early as May, when Mr. Robert Mueller was appointed as Special Counsel in charge of investigating allegations of Russian interference in the US election, Deputy Director of the US Federal Bureau of Investigation (FBI) Andrew McCabe told a number of senior officials in the unit that they should prepare mentally to become witnesses.

Special counsel in charge of investigating allegations of Russian interference in the US election Robert Mueller. Photo: Getty.

The investigation Mr. Mueller inherited initially centered on whether President Donald Trump's campaign sought help from the Russian government to defeat Democratic rival Hillary Clinton in the 2016 election.

When it came to Mr. Mueller, the investigation was expanded to the issue of whether Mr. Donald Trump obstructed justice when the US President was said to have asked FBI Director James Comey to stop the Russia investigation and “let go” of his confidant – National Security Advisor Michael Flynn – in a private meeting on January 27, 2017. At that time, Mr. Donald Trump knew Mr. Flynn was under criminal investigation.

Once Mr. Mueller forms a grand jury, the Special Counsel could have expanded powers to gather evidence and subpoena witnesses for the investigation.

More witnesses than Donald Trump thinks

President Donald Trump and his supporters have long argued that it would be difficult, if not impossible, for Special Counsel Robert Mueller to bring charges of obstruction of justice against Donald Trump. They argue that the case would be based on the word of one man [former FBI Director James Comey] against the President of the United States.

But in fact, Mr. Comey regularly communicated with six senior FBI officials, especially after tense talks with President Donald Trump, about the Russia investigation.

According to Vox, Mr. Comey had at least eight conversations with subordinates, one-on-one or in meetings, about the meeting with President Donald Trump, some of which lasted for hours. Including Mr. Comey, as many as 10 or more law enforcement officials will be questioned as part of the investigation.

Among those rumored to be witnesses against President Donald Trump are six top FBI officials. They include Deputy Director McCabe himself; former Director Comey’s chief of staff, Jim Rybicki; FBI General Counsel James Baker; FBI Deputy Director David Bowdich, the agency’s third-highest-ranking official; and finally, McCabe’s legal adviser, FBI National Security Division Chief Carl Ghattas.

Chief of Staff Rybicki was even present with Mr. Comey during the phone call in which President Donald Trump asked the FBI Director whether he was personally under investigation. According to Mr. Comey’s testimony before the US Congress, Mr. Rybicki directly overheard at least half of that conversation from the former FBI Director.

Meanwhile, Mr. Baker was a constant presence in discussions about whether to inform the Justice Department that the President had pressured Mr. Comey to close the Flynn investigation. Mr. Baker was also the most vocal opponent in discussions about whether Mr. Comey or the Justice Department should accede to President Donald Trump’s request to disclose whether the investigation focused on him.

More witnesses, more evidence

Also according to Vox's source, FBI General Counsel Baker was the one who recorded conversations with Mr. Comey and other senior officials about President Donald Trump's efforts to obstruct the FBI investigation.

Thus, in addition to Mr. Comey's notes about his conversation with President Donald Trump, Mr. Baker's notes can also be used as evidence.

It is unclear whether any additional FBI officials have relevant records.

FBI agents are experienced in testifying in sensitive investigations of high-level figures. The FBI itself is an agency widely respected and trusted by the American public, so their testimony carries considerable weight.

The FBI's witness list, along with evidence such as notes from Mr. Comey and other bureau officials, would significantly strengthen the case for obstruction of justice.

Donald Trump's confidants may also be summoned.

Attorney General Jeff Sessions, Deputy Attorney General Rod Rosenstein and a third senior Justice Department official are also expected to testify.

Because Mr. Comey testified before the US Congress that in a private meeting with Mr. Sessions on another matter, he asked the Attorney General to “prevent direct conversations” between him and the President. Comey made it clear that leaving him alone with Donald Trump was “inappropriate and should not happen again.” However, according to Comey’s testimony, Sessions did not respond and his body language showed “an inability or unwillingness to do anything.”

Mr. Comey also testified that he expressed similar concerns to Mr. Rosenstein.

In his subsequent congressional testimony, Sessions all but admitted that Mr. Comey was uncomfortable “interacting” with the President.

Investigators and experts believe their testimony will likely side with former FBI Director Comey and hurt President Donald Trump more.

“In cases involving high-level officials, investigators will have to question anyone who was involved in the conversations,” said Sam Buell, a former New York federal prosecutor. “That doesn’t mean they’re going to be a witness in court, but at the investigative level, you want their story to be fully uncovered.”