(CNN) A key memo in special counsel Robert Mueller's investigation accused Paul Manafort of allegedly colluding with the Russian government when he was then-presidential candidate Donald Trump's campaign chairman in 2016 — but a criminal charge never came.

The accusation is one of the most conspicuous and direct, unresolved allegations Mueller made publicly over the last 22 months and is a lingering question now that he has submitted his report to Attorney General William Barr.

The accusation was revealed last April, when prosecutors submitted to a court a still-largely redacted memo then-Acting Attorney General Rod Rosenstein wrote to Mueller.

The memo , from August 2017, outlined exactly what Mueller should investigate. Numerous bullet points were blacked out — more than a page in full — yet two bullet points about Manafort were publicly visible.

Rosenstein directed Mueller to investigate "allegations that Paul Manafort committed a crime or crimes by colluding with Russian government officials with respect to the Russian government's efforts to interfere with the 2016 election for President of the United States, in violation of United States law."

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