Washington(CNN) The White House has accused special counsel Robert Mueller's team of playing politics with the investigation and wildly straying from their mission in a letter sent to Attorney General William Barr last month and released Thursday afternoon.

In the five-page letter, a top White House lawyer, Emmet Flood, raised several concerns with the substance and format of Mueller's report, which did not establish a criminal conspiracy between Trump's campaign and the Russians. Mueller also unearthed substantial evidence of obstruction by Trump, but the report didn't say whether the President should be prosecuted.

Flood slammed Mueller's approach to the obstruction investigation, saying "the special counsel and his staff failed in their duty to act as prosecutors and only as prosecutors."

Even though current Justice Department guidelines say a sitting president cannot be charged, Flood wrote that Mueller needed to "either ask the grand jury to return an indictment or decline to charge the case."

"The (special counsel) instead produced a prosecutorial curiosity -- part 'truth commission' report and part law school exam paper," Flood wrote.

"Far more detailed than the text of any known criminal indictment or declination memorandum, the report is laden with factual information that has never been subjected to adversarial testing or independent analysis," he added.

Trump, however, has been touting the report and Barr's analysis as proof he was exonerated. "No Collusion - No Obstruction!" he tweeted April 18 and has repeated several times.

The letter is dated April 19, one day after the Justice Department released the redacted report to the public.

In the report, Mueller directly explained how those internal Justice Department rules against indicting a president had a major impact on his internal deliberations. In effect, Mueller framed his entire obstruction investigation around the notion that he couldn't bring any charges against Trump even if he found ironclad evidence against him, but wanted to preserve the evidence and included references to Congress' unique constitutional role to hold a president accountable.

Mueller explained in his report that he saw the effort in part as preserving details because the President "does not have immunity after he leaves office" and that his team "conducted a thorough factual investigation in order to preserve the evidence when memories were fresh and documentary materials were available."

Another part of the letter explains the White House argument that Mueller overstepped his role by providing a "road map" that Congress could use to initiate impeachment proceedings. Mueller never said this directly in his report, but some Democrats and commentators have said that the report could give Congress what it needs to take the next steps against Trump.

"Under a constitution of separated powers, (Justice Department officials) should not be in the business of creating 'road maps' for the purpose of transmitting them to (Congressional) committees," Flood wrote.

The letter criticizes Mueller's decision to document nearly 200 pages of extraordinary details from its obstruction investigation. Justice Department regulations require Mueller to submit a "confidential report" to the attorney general explaining decisions whether not to charge people under investigation. The regulations don't impose limits on the length or detail of the report.

Essentially, the White House letter argues that Mueller's team was playing politics when they specifically stated in the report that it "does not exonerate" Trump of obstruction of justice.

"The (special counsel's) inverted-proof-standard and 'exoneration' statements can be understood only as political statements, issuing from persons (federal prosecutors) who in our system of government are rightly expected never to be political in the performance of their duties," Flood said, echoing Trump's longstanding position that Mueller's team is biased.

With an eye toward upcoming battles with House Democrats, the letter also makes clear that the White House wants to preserve all executive privilege for all future proceedings. Just because the President did not assert privilege for the report doesn't mean he will not do so in the future. Flood also aimed to draw a distinction between cooperating in a criminal investigation and a congressional investigation led by the House Democrats.

"The President therefore wants the following features of his decision to be known and understood," Flood wrote. "His decision not to assert privilege is not a waiver of executive privilege for any other material or for any other purpose."