"So many questions," Comey tweeted, alongside a photo of himself in a forest, looking upwards while surrounded by tall trees.
Mueller was appointed special counsel of the Russia probe shortly after President Trump had fired Comey in May 2017.
Hours before Comey's tweet, Attorney General William Barr released a four-page letter detailing what he called the "principal conclusions" of Mueller's investigation into possible collusion between President Trump's 2016 campaign and Russia.
The special counsel's office, according to Barr, "did not find that the Trump campaign, or anyone associated with it, conspired or coordinated" with Russians who worked on those hacking efforts "despite multiple offers from Russian-affiliated individuals to assist the Trump campaign."
Following a "thorough factual investigation" into whether Trump possibly obstructed justice, the special counsel's office "did not draw a conclusion" because it was unclear what constituted obstruction of justice.
Mueller "recognized" that the lack of evidence that Trump was involved in collusion would undercut any obstruction case — which would depend on showing a corrupt intent by the president. The investigation stated that "while this report does not conclude that the President committed a crime, it also does not exonerate him."
Trump later called the investigation "an illegal takedown that failed."
“I have no idea whether the special counsel will conclude that Mr. Trump knowingly conspired with the Russians in connection with the 2016 election or that he obstructed justice with the required corrupt intent," Comey wrote. "I also don’t care."
“I care only that the work be done, well and completely.”
Fox News' Andrew O'Reilly contributed to this report.