"I'm not doing it intentionally," Muhammad said after being admonished by Malvo, the judge and prosecutors. "I apologize, Mr. Malvo."
Muhammad, who is representing himself, also questioned Malvo about his mental health, referring to his 2003 insanity plea in Virginia for one of the sniper killings.
"Who decided you was insane? ... How many doctors said you was insane?" Muhammad asked Malvo.
"They said I was indoctrinated," Malvo responded.
Muhammad's questions came a day after Malvo's testimony provided an insider's account of the October 2002 sniper shootings that left 10 people dead. Malvo called Muhammad a coward who "made me a monster."
Muhammad, 45, and Malvo, 21, already have been convicted in Virginia for a sniper murder there. Muhammad received a death sentence while Malvo was given a life term.
Prosecutors in Maryland have said they are pursuing a second trial in case the Virginia conviction is overturned on appeal and to seek justice in Montgomery County, where six of the 10 killings happened.
On Tuesday, Muhammad said he had told jurors at the trial's opening that "John Allen Muhammad and Lee Boyd Malvo are innocent and I'm going to prove it."
Malvo remained unmoved by Muhammad's statement. When Muhammad asked Malvo if he believed Muhammad would indeed exonerate the pair, Malvo responded no.
Later, Malvo became agitated as Muhammad pressed him on a variety of details, as Muhammad has done to others throughout the trial. Asked repeatedly by Muhammad how often their Bushmaster rifle was stored in a duffel bag, Malvo finally blurted out: "The weapon's been in the bag plenty of times, Muhammad."
Muhammad questioned Malvo about inconsistent statements he has given about the shootings. Shortly after the two were arrested, Malvo took responsibility for all 13 sniper shootings.
On the witness stand Tuesday, Malvo said Muhammad fired 10 shots and Malvo three. Muhammad then quizzed Malvo on whether he had been improperly convicted in 2003 for one of the killings in which Malvo now says Muhammad pulled the trigger.
"No, I'm not innocent. I took part in that shooting. I'm an accomplice," he said.
Malvo's daylong testimony included a number of startling admissions.
Malvo said Muhammad's initial plan called for six shootings a day for 30 days, to be followed by a second phase that would include bombings of school buses and children's hospitals.
He said Muhammad told him "we're going to go to the Washington, D.C., area and we're going to terrorize these people." Malvo said Washington was chosen because that was where Muhammad's ex-wife had taken the couple's children. The plan ultimately called for taking the children to Canada.
Malvo testified he became so despondent on hearing Muhammad's plans that he played Russian roulette while sobbing in a bathtub, pulling the trigger several times before breaking down.
Malvo said Muhammad altered the plans midway through the sniper spree after he and Malvo nearly had a falling out.
Muhammad told Malvo they would obtain a $10 million payment in exchange for stopping the killings, and then recruit 140 homeless children to a compound in Canada where they would trained to "continue the mission" — namely, a prolonged terror campaign against America, Malvo said.
Malvo said he "just latched onto that" and believed it because Muhammad had always been truthful with him.
At the end of several hours of questioning, prosecutor Katherine Winfree asked Malvo what he now thinks of Muhammad. "I think he is a coward," Malvo said.
Malvo then glared at Muhammad: "You took me into your house and you made me a monster," he said.