Malvo, 18, is being held at the Fairfax County jail on capital murder charges in the Oct. 14 shooting of Linda Franklin, 47, outside a Home Depot store.
Malvo and John Allen Muhammad, 42, have been linked to 20 shootings last year, including 13 deaths, in Virginia, Maryland, Georgia, Alabama, Louisiana and the District of Columbia. They face trial in Virginia first.
Jail officials disciplined Malvo last month for writing "Muhammad" on his cell floor and drawing on his shoes. This time, jail officials said, Malvo sent a letter to another inmate, violating a policy prohibiting such communications.
The letter was mailed out of the jail to a phony address and used the other inmate's name as the return address so it would be sent back to him to circumvent the rule, Maj. James Whitley said.
In the letter, Malvo wrote of a deputy that "someone should ensure that boy is fatally injured."
Malvo is accused of violating prison rules by threatening a jail employee and writing another inmate a letter. He faces an administrative hearing next week, with his lawyer present.
If he is found guilty of the violations, he could be placed in disciplinary segregation, as he was after last month's violations. His mattress would be removed for 16 hours each day, he would lose visiting and phone privileges, and he would be fed only the jail's "vegetarian loaf" — a mixture of eggs, cheese and vegetables.
Fairfax Sheriff Stan G. Barry said the incident was not serious enough to warrant criminal charges.
Mark Petrovich, one of Malvo's lawyers, said the letter was not a direct threat. "It's not like he was trying to intimidate anyone," he said.