Federal prosecutors in the Southern District of New York are seeking to impose stricter bail conditions on Sam Bankman-Fried, the former CEO of crypto exchange FTX, who is facing charges including wire fraud and money laundering.

The prosecutors have revealed that Bankman-Fried, also known as SBF, has been messaging the general counsel of FTX US via Signal, a messaging app known for its emphasis on security and privacy.

The prosecutors argue that these messages are “suggestive of an effort to influence a witness’ potential testimony” and are asking the judge overseeing the case to prevent SBF from communicating with former employees and to prohibit him from using Signal.

Additionally, they claim that SBF continues to receive visitors even under house arrest, including author Michael Lewis who is reportedly writing a book about the crypto entrepreneur.