The Department of Justice confirmed to INSIDER that Matthew Whitaker, the acting attorney general, would assume control of the FBI's Russia investigation led by the special counsel Robert Mueller.

Whitaker, seen by many as a hardline loyalist to President Donald Trump, has written and commented extensively expressing his belief that Mueller needs to be reined in.

Trump's decision to oust Jeff Sessions as attorney general on Wednesday and replace him with Whitaker sparked concern among lawmakers and national-security experts who described the move as an attempt to gut the Russia investigation.

"Why is the President making this change and who has authority over Special Counsel Mueller's investigation?" Democratic Rep. Jerry Nadler tweeted. "We will be holding people accountable."

Democrats regained control of the House of Representatives on Tuesday and are widely expected to function as a check on Trump's attacks on the DOJ and the FBI.

The Department of Justice confirmed to INSIDER on Wednesday that Matthew Whitaker, the acting attorney general replacing Jeff Sessions, would assume oversight of the FBI's Russia investigation led by the special counsel Robert Mueller.

"The Acting Attorney General is in charge of all matters under the purview of the Department of Justice," a DOJ representative said in an email.

Whitaker, a former US attorney from the Southern District of Iowa, is viewed by many as a staunch Trump loyalist.

The New York Times reported in September that the White House chief of staff, John Kelly, once described Whitaker as the West Wing's "eyes and ears" in the DOJ, which is investigating whether the Trump campaign colluded with Russia to tilt the 2016 election in his favor.

Whitaker appears to share the White House's skepticism of Mueller and the Russia investigation. Shortly before he was hired as Sessions' chief of staff last year, Whitaker wrote in an op-ed article for CNN that Mueller had "come up to a red line in the Russia 2016 election-meddling investigation that he is dangerously close to crossing."

Whitaker added that his concerns stemmed from reports that the special counsel was investigating the Trump Organization's financial records. Mueller's scrutiny of Trump's finances "falls completely outside of the realm" of his appointment, Whitaker wrote.

But in a letter outlining the scope of Mueller's appointment last year, Deputy Attorney General Rod Rosenstein gave Mueller broad authority to not only investigate "any links and/or coordination between the Russian government and individuals associated" with Trump's campaign, but examine "any matters that arose or may arise directly from the investigation."

Rosenstein also gave Mueller the power to investigate "any other matters within the scope" of the law outlining a special counsel's jurisdiction, including perjury, obstruction of justice, destruction of evidence, and intimidation of witnesses.

Sessions' ouster on Wednesday sparked widespread concern from lawmakers and national-security experts who described it as an attempt to gut Mueller's investigation.

"Given his previous comments advocating defunding and imposing limitations on the Mueller investigation, Mr. Whitaker should recuse himself from its oversight for the duration of his time as acting attorney general," Senate Minority Leader Chuck Schumer said in a statement.

House Democrats, who will have a majority in the chamber following Tuesday's midterm elections, also sounded the alarm.

Rep. Jerry Nadler, the ranking member of the House Judiciary Committee, said in a tweet that Americans "must have answers immediately as to the reasoning behind" Trump's decision to remove Sessions.

"Why is the President making this change and who has authority over Special Counsel Mueller's investigation?" Nadler added. "We will be holding people accountable."

Jeffrey Cramer, a longtime former federal prosecutor who spent 12 years at the Justice Department, told INSIDER in a text that it was "safe to say this news wasn't shocking to Mueller."

"Perhaps the timing, but everyone knew Sessions' tenure was limited," Cramer added. "We should see grand jury action or a report soon. Whitaker could try to slow-walk future efforts to sit on a report. But it's naive to think the new Democratic House of Representatives just sits there and watches."