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.

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