Washington (CNN) Hundreds of former Justice Department officials said in an open letter released Monday that President Donald Trump would be facing multiple felony charges for obstruction of justice stemming from the Russia investigation if he were not President.

The letter posted online by Justice Department alumni, who served under presidents from both parties, said the report from special counsel Robert Mueller contained repeated instances of Trump committing obstruction of justice, and that he would have been charged with obstruction if he was not protected as President by an opinion from the Justice Department's Office of Legal Counsel that Mueller cited.

"We believe strongly that, but for the OLC memo, the overwhelming weight of professional judgment would come down in favor of prosecution for the conduct outlined in the Mueller Report," the letter read.

The group's assertion comes as controversy over Mueller's findings and the attorney general's reaction to them grows -- particularly regarding whether the President obstructed justice. Since Mueller released a redacted version of his report to the public on April 18, Attorney General William Barr has faced intense criticism from Democrats who say he downplayed the findings, while Mueller has been attacked by the White House.

Mueller, in the half of his report about the obstruction investigation, laid out 10 situations in which he investigated Trump's actions and motivations, including the firing of former FBI Director James Comey and Trump's pressure on then-White House counsel Don McGahn to remove Mueller from his job.

In many of those situations, Mueller found that Trump had taken obstructive acts that could have hurt ongoing investigations and intended to disrupt the investigators at times because of his own personal motivations, like silencing questions about his 2016 presidential election victory.

Instead of deciding whether to prosecute, Mueller said he would not exonerate Trump and pointed to the Justice Department guidance that a President could not be indicted while in office as a reason for not reaching a conclusion.

Instead, Barr made the decision alongside Deputy Attorney General Rod Rosenstein not to prosecute Trump. Barr told Congress and the public before Mueller's report was released that the Justice Department wouldn't bring the case because the evidence of an underlying crime was lacking and didn't find conduct that showed obstruction.

Yet more than 600 former Justice Department officials and prosecutors as of Tuesday morning disagree.

The letter was posted to Medium and said it was being updated by the group Protect Democracy, a nonprofit group that has combated the Trump administration. The nonprofit political advocacy group said a group of former prosecutors contacted the organization last week, and Protect Democracy then solicited other signers for the letter.

The letter was signed by officials from a wide-range of backgrounds, and included former US attorneys and other top officials from both parties. Almost 50 said they had served in the Justice Department for three decades or more. More than 30 signers said they had worked there during Trump's presidency, including two US attorneys who left their leadership posts shortly after Trump became president. In all, more than 20 former US attorneys -- who typically make prosecutorial decisions for their districts -- have signed the letter.

Sen. Richard Blumenthal, a former US attorney in DC and a prominent member of the Judiciary Committee panel that questioned Barr last week, also signed the letter.

The Washington Post, which previously reported on the letter, said signatories to the letter included officials whose time in government included every administration since President Dwight Eisenhower.

This wasn't the first time that a cadre of former Justice Department officials have spoken out about Trump and the administration's approach to the law -- or even about Mueller's findings.

At the end of April, a bipartisan group of well-known lawyers, including two former acting attorneys general and George Conway, husband of one of Trump's top advisers, said Trump's actions to obstruct the Mueller investigation amounted to "high crimes and misdemeanors" that Congress now should investigate.

Earlier in the Trump presidency, Justice Department alumni posted an open letter defending Mueller and the agency as Trump publicly attacked them.

This story has been updated.