Special Counsel Robert Mueller said William Barr’s summary of the Russia investigation lacked context and detail and wrote a letter to the attorney general in late March to express his objections, reports said on Tuesday.

Mueller told the attorney general that the four-page letter presented to Congress that described the conclusions of the investigation “did not fully capture the context, nature, and substance” of the extensive investigation, according to The Washington Post.

The letter from Mueller was sent on March 27, following Barr’s release of a summary that said Mueller had not established a conspiracy between the Trump campaign and Russia to interfere with the 2016 presidential election. Barr’s summary also said Mueller had not found sufficient evidence for an obstruction of justice charge.

A spokesperson for the Justice Department acknowledged that the letter was sent but insisted that it did not dispute the facts of the summary.

“The special counsel emphasized that nothing in the attorney general’s March 24 letter was inaccurate or misleading,” the spokesperson, Kerri Kupec, told The New York Times. “But he expressed frustration over the lack of context and the resulting media coverage regarding the special counsel’s obstruction analysis.”

The letter comes to light as Barr prepared to testify before the Senate Judiciary Committee about the investigation on Wednesday.

Democrats reacted on Tuesday by writing the Office of Inspector General in the Justice Department to demand an investigation into Barr’s conduct overseeing the probe.