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KEY POINTS A document that is sealed from public view was placed in a New York federal court vault in a criminal case against President Donald Trump's former lawyer, Michael Cohen.

The filing comes a week after Cohen was sentenced to three years in prison for financial crimes and for campaign finance violations related to hush money payments to porn star Stormy Daniels and Playboy model Karen McDougal.

Cohen also pleaded guilty to lying to Congress about an aborted plan to build a Trump Tower in Moscow. That case was filed by special counsel Robert Mueller, who is continuing to investigate Trump's presidential campaign and its ties to Russians.

President Donald Trumps former attorney Michael Cohen arrives at US Federal Court in New York on December 12, 2018, where he is expected to be sentenced after pleading guilty to a number of charges. Corey Sipkin| AFP | Getty Images

What's in the vault? Someone on Wednesday filed a document saying something in connection with a criminal case against President Donald Trump's former personal lawyer, Michael Cohen. But who filed it and what it says are a mystery. The document, which is sealed from public view, was "placed in vault" at U.S. District Court in Manhattan, according to a docket entry. The filing came a week after Cohen, 52, was sentenced to three years in prison for his guilty pleas in cases brought separately by federal prosecutors in New York, and by the office of special counsel Robert Mueller. The sealed document was filed in a case brought against Cohen by the U.S. Attorney's Office for the Southern District of New York. In that case, Cohen pleaded guilty to tax evasion, making false statements to a bank, and to campaign contribution crimes related to the payments of hush money to two women who claim they had affairs with Trump. WATCH: Trump underreported payments to Cohen in disclosures, a potential violation

In the case filed by Mueller, Cohen admitted lying to Congress about the extent of Trump's knowledge of a plan to build a Trump Tower in Moscow, and about when that plan died. The sealed document does not appear on the docket of that case. Neither Cohen's lawyer Guy Petrillo nor a spokesman for the federal prosecutors in New York immediately responded to emails asking who filed the document or what it says. One question that has loomed since Cohen was sentenced is whether he will now try to get his prison term reduced by offering prosecutors additional cooperation beyond what he already has given them. Cohen is due to surrender to prison on March 6. Before his sentencing, Cohen met seven times with investigators from Mueller's office to assist them with their ongoing investigations, which included probing possible collusion by the Trump campaign with Russian agents' interference in the 2016 election. He also met twice with prosecutors from the Southern District of New York. Legal experts have said those prosecutors may have grounds to charge Trump with campaign finance crimes for allegedly directing Cohen to facilitate payments to the two women in order to keep their claims out of the public eye before Election Day 2016.