Federal prosecutors are attempting to bar indicted FTX co-founder Sam Bankman-Fried from using encrypted messaging software, citing efforts that may “constitute witness tampering,” according to a letter filed in Manhattan federal court Friday.

Bankman-Fried reached out to the “current General Counsel of FTX US who may be a witness at trial,” prosecutors said. Ryne Miller, who was not identified by name in the government filing, is the current counsel for FTX US, and a former partner at Kirkland & Ellis.

The government claims that Bankman-Fried wrote to Miller via Signal, an encrypted messaging app, on Jan. 15, days after bankruptcy officials at crypto exchange disclosed the recovery of more than $5 billion in FTX assets.

“I 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,” Bankman-Fried allegedly told Miller.

Bankman-Fried has also been in contact with “other current and former FTX employees,” the filing said. Federal prosecutors allege that Bankman-Fried’s request suggests an effort to influence the witness’s testimony, and that Bankman-Fried’s effort to improve his relationship with Miller “may itself constitute witness tampering.”

Both Miller and a representative for Bankman-Fried declined to comment.

In restricting Bankman-Fried’s access to Signal and other encrypted messaging platforms, the government cites a need to “prevent obstruction of justice.” Federal prosecutors claim that Bankman-Fried directed Alameda and FTX through Slack and Signal, and ordered his employees set communications to “autodelete after 30 days or less.”

Citing previously undisclosed testimony from ex-Alameda CEO Caroline Ellison, the government claimed that Bankman-Fried indicated “many legal cases turn on documentation and it is more difficult to build a legal case if information is not written down or preserved.” Ellison pled guilty to multiple charges of fraud and has been cooperating with the U.S. Attorney’s efforts to build a case against Bankman-Fried.

Bankman-Fried pled not guilty to eight charges in connection with the collapse of his multibillion-dollar crypto empire, FTX. He is due in federal court in October, after being released on $250 million bond.