Circuit Judge Jane Marum Roush had refused to let defense attorneys show or read the letter to the jury, saying it was hearsay. The Post printed the letter's text Thursday and included excerpts in Malvo's handwriting. The newspaper did not say how it obtained the letter.
In court Thursday, the judge questioned the prosecutors and defense attorneys in the case, and all told her they did not leak the letter.
Roush also said she was "disturbed" by the daily news conferences defense attorneys had been holding after court.
"I'm going to enter a gag order because I am increasingly disturbed by this. I think it's an attempt to reach the jurors or the jurors' families," Roush said. "No more talking to the papers, no more having press conferences."
Defense lawyer Michael Arif had argued in court Wednesday that the jury should be allowed to see the letter because it showed Malvo's gloomy state of mind just months before the sniper spree that left 10 people dead in and around the nation's capital.
After court, defense attorneys said there may be other ways to get the letter into evidence, but they did not elaborate.
Malvo wrote the letter to LaToria Williams, a teenage niece of convicted sniper mastermind John Allen Muhammad, during a visit to Muhammad's family in Baton Rouge, La., just weeks before the October 2002 sniper attacks that killed 10 people in the Washington area.
Williams testified outside the jury's presence that she was scared after reading the letter because "he said that he was a ticking time bomb."
Lawyers for the 18-year-old Malvo, on trial in the shooting death of an FBI analyst, are pursuing an insanity defense, contending that Muhammad brainwashed him and turned him into a killer. Muhammad, 42, was convicted last month of capital murder in another of the deadly Washington-area sniper shootings, and a jury recommended the death sentence. Malvo could also faces the death penalty if convicted.
In other testimony Wednesday, Kip Berentson said he was so unnerved by the soldier in his platoon suspected of throwing a grenade into an Army tent in 1991 that he scribbled the man's name and dog-tag number on a piece of paper he still keeps in his wallet.
"I considered him a threat," Berentson said of Muhammad.
Berentson, who was Muhammad's platoon sergeant, testified that no one was injured when an incendiary grenade was lobbed into a tent in the middle of the night, just before the United States began the ground war in the Persian Gulf War.
Muhammad was on the ground, coughing, after the incident, but a medic who examined him thought he was faking, Berentson said. Investigators weren't able to prove that Muhammad threw the grenade, but he was transferred to another platoon, Berentson said.
Berentson also read a portion of Muhammad's military record in which a supervisor wrote that he counseled Muhammad for poor performance and Muhammad answered with a threat: "Brother to brother, back off or you'll be the first who will be slaughtered."