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.

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