A spokesman for Mr. Bankman-Fried did not immediately respond to a request for comment.

Mr. Bankman-Fried built FTX into one of the biggest cryptocurrency exchanges in the world before the company filed for bankruptcy in November. He was arrested in December at his home in the Bahamas, where FTX was based, and then extradited to the United States to face the criminal charges. Judge Kaplan granted him bail under highly restrictive conditions, confining him to his parents’ home near the campus of Stanford University in Northern California.

Mr. Bankman-Fried has pleaded not guilty to the charges against him.

From his home confinement, Mr. Bankman-Fried has entertained visitors, including the author Michael Lewis, who is writing a book about him. He has also started mounting a defense, writing posts on Substack that detail his version of the events leading up to FTX’s collapse.

According to Friday’s filing, Mr. Bankman-Fried wrote to the general counsel of FTX U.S. on Jan. 15, saying he would “really love to reconnect and see if there’s a way for us to have a constructive relationship, use each other as resources when possible, or at least vet things with each other.”

Prosecutors said the witness “has firsthand knowledge of the defendant’s conduct during the charged conspiracies” and participated in communications on Signal and the messaging system Slack with a small group of insiders the month that FTX collapsed. Mr. Bankman-Fried has also contacted other current and former FTX employees, according to the filing.

The document does not identify the witness by name. The general counsel of FTX U.S. is Ryne Miller, according to his LinkedIn page.