Matthew Hale, the leader of a racist cult who was convicted last year of plotting to kill Judge Joan Lefkow, gave cryptic messages to his mother, Evelyn Hutcheson, and asked her to pass them on.
Judge Lefkow's mother and husband were shot dead in Chicago last week, a double murder which the police suspect may be linked to Hale's supporters in the white supremacist movement.
Hale, who is awaiting sentencing, is being held at the Metropolitan correctional centre in New York under measures for suspected terrorists introduced after the September 11th attacks.
His only contacts with the outside are fortnightly visits by his parents, who are allowed to spend an hour each with him in the presence of an FBI officer.
Hale's lawyer Glenn Greenwald told the New York Times that a few months ago Hale's mother had asked him to pass on a cryptic message to one of her son's supporters.
"She said she didn't know what the message meant, but she was going to read it to me verbatim because Matt made her write it down when she visited him," Mr Greenwald said. "It was two or three sentences that were very cryptic and impossible to understand."
Mr Greenwald said he had refused to pass on the message, which he described as "a caricature of what a coded message would be".
An official at the centre told Hale's parents on Monday that they were no longer allowed to visit him or speak to him.
His father Russell Hale, a retired police sergeant, told the Associated Press that he was shocked by the ban. "I was just so stunned that I didn't think to ask why."
Last week Hale issued a statement through his mother denying being involved in the double murder, and Mr Greenwald suggested that their contact with him might have been severed because Mrs Hutcheson issued the denial.
"She's not allowed to disseminate any messages," he said.
On the same day the police issued two photofit pictures of men wanted in connection with the murder.
When the police asked Mrs Hutcheson last week whether she had received any coded messages from her son, she said it was "the dumbest thing I've ever heard".
On Monday she said the message was intended solely for Mr Greenwald. "I said, 'Matt, this doesn't make sense to me.' He said, 'Greenwald will understand; ask him to read the letter'."