WASHINGTON -- Eager to begin another assault on the credibility of those investigating him, President Donald Trump said Tuesday he wants to know "what is taking so long" with an internal Justice Department investigation that is expected to cast a harsh spotlight on the FBI.

The tweet underscores how the White House and Trump's legal team view the upcoming watchdog report as a way to damage the reputation of fired FBI director James Comey -- and, by extension, discredit special counsel Robert Mueller's investigation into possible ties between the Trump campaign and Russia.

The report is expected to sharply criticize Comey's handling of the Hillary Clinton email investigation, which the White House initially cited as the rationale for firing the FBI chief last year. Trump and his allies continue to believe any censures of Comey would undercut his value as a witness against the president and back up the decision to dismiss him last May.

Democrats have long criticized Comey's decisions -- including an unusual news conference announcing his recommendation against criminal charges for Clinton, and his notification to Congress days before the election the investigation was being reopened -- as having broken from protocol in ways damaging Clinton's election prospects. An inspector general report, expected within days, is likely to find fault with some of those same actions.

Trump, nonetheless, has eagerly awaited the report in hopes any damning conclusions about Comey and the FBI will give him additional fodder to attack federal law enforcement officials. Part of the defense team strategy has been to discredit Mueller's investigation by condemning Comey, who has testified publicly about his interactions with the president and who set in motion Mueller's appointment by releasing to a friend a private memo he wrote documenting a pivotal encounter with Trump.

The president's allies have come to believe pitting Trump's trustworthiness against Comey's is an argument they can win, according to three people close to the White House but not authorized to discuss private conversations. They have repeatedly attempted to tarnish the reputation of the special counsel probe, of late fixating on the claims it was launched on the backs of memos they believe Comey improperly disclosed.

Comey has said he was authorized as a private citizen to share one of his memos with a friend, who later disclosed details of it to reporters, and has said the document did not contain classified information. In the memo, Comey described being asked by Trump to consider ending an investigation into former White House national security adviser Michael Flynn.

Rudy Giuliani, one of the president's lawyers, told The Associated Press in recent days he believed the report would be damaging to Comey's reputation. A separate report on fired FBI Deputy Director Andrew McCabe accused the former No. 2 official of having misled internal investigators about his role in an October 2016 news media disclosure, heightening anticipation the upcoming report would pile onto that criticism.

"This is going to be the final nail in his coffin," Giuliani said. "This guy has already proven to be a leaker and liar, and we believe the report is going to make that plain."

Trump took to Twitter Tuesday to vent publicly what he has been saying privately for days: he wants the report released as soon as possible and is fearful his enemies in the Justice Department may attempt to doctor it to protect Comey, according to an outside adviser and a White House official who were not authorized to speak publicly about private conversations.

"What is taking so long with the Inspector General's Report on Crooked Hillary and Slippery James Comey," the president tweeted. "Numerous delays. Hope Report is not being changed and made weaker! There are so many horrible things to tell, the public has the right to know. Transparency!"

Though a draft has been done for weeks, people whose actions have been scrutinized in it -- and their lawyers -- have been permitted to review a copy and propose tweaks. That review is standard, and there is no indication its release has been delayed for any unusual or improper reasons.