More than 400 former Justice Department officials are objecting to President Donald Trump’s appointment of Matthew Whitaker as acting attorney general.

“Because of our respect for our oaths of office and our personal experiences carrying out the Department’s mission, we are disturbed by the President’s appointment of Matthew Whitaker to serve as Acting Attorney General,” the former officials wrote in the signed statement, which was first published by BuzzFeed News on Tuesday. “We know that overseeing the Department of Justice is one of the most important roles in our government.”

This statement from hundreds of former DOJ officials is the latest public opposition against Trump’s decision to appoint Matthew Whitaker as acting attorney general to replace Jeff Sessions, who resigned as attorney general at the president’s request after the midterm elections.

Whitaker was serving as Session’s chief of staff, and Trump handpicked him for the role using a provision of the Federal Vacancies Reform Act of 1998, rather than leaving Rod Rosenstein, the deputy attorney general and No. 2 at the Department of Justice, in charge.

Whitaker was also an odd choice for another reason: Prior to joining the administration, he had been a vocal and unabashed critic of special counsel Robert Mueller’s investigation into Russian interference in the 2016 presidential election. Now Whitaker, as acting AG, assumes control of the probe.

This had led to some calls, mostly from Democrats, that Whitaker recuse himself from the probe. A number of lawsuits have also been filed challenging Whitaker’s legitimacy, many of them arguing that his appointment is unconstitutional because he never received Senate confirmation and other Senate-confirmed officers were available to fill the interim role.

Others have made the argument that the Justice Department’s order of succession should trump the Vacancies Reform Act, meaning Rosenstein should have been tapped to lead until a suitable replacement was nominated and Senate-confirmed.

The signatories to this recent statement echo those arguments, calling on Trump to formally nominate an attorney general to face Senate confirmation and, in the interim, replace Whitaker with “the Senate-confirmed official who is next in the line of succession by operation of federal law.”

“Mr. Whitaker has not been confirmed by the Senate, his qualifications to be the nation’s chief law enforcement officer have not been publicly reviewed, and he has not been fully vetted for any potential conflicts of interest,” the former officials write. “While we know that there are thousands of dedicated public servants now at the Department who will do their utmost to protect its mission and reputation, it falls to all of us to ensure that the Department’s role in maintaining the rule of law is not undermined or tainted.”

The majority of people who signed the letter are former career Justice officials who served in both Democratic and Republican administrations, reported BuzzFeed. The remaining signatories are former Justice Department appointees, the majority of whom served in Democratic administrations.

Read the full letter here.