Convicted Washington sniper John Allen Muhammad has questioned his young accomplice Lee Boyd Malvo at a second murder trial in Maryland.
Malvo (left) and Muhammad (standing) had a testy exchange
Muhammad, 45, acting as his own lawyer, was admonished several times by the judge for his line of questioning.
He is accused of six counts of murder in Montgomery County, just outside Washington DC, in 2002.
In 2004, he was sentenced to death by a court in neighbouring Virginia after he was convicted of one fatal shooting.
Malvo, now 21, was jailed for life in Virginia. He agreed to testify against Muhammad at the trial in Rockville, Maryland and to plead guilty.
Muhammad, dressed in a grey suit, conducted his cross-examination, appearing at times aggressive, at times gentler.
At one point, he called Malvo "son".
"I would prefer you address me by my name," Malvo replied.
Muhammad apologised, only to do it again.
Muhammad questioned the younger man about his mental health, referring to his insanity plea lodged at an earlier trial.
"Who decided you was insane... How many doctors said you was insane?" Muhammad asked Malvo.
Malvo was 17 at the time of the Washington killings
"They said I was indoctrinated," said Malvo.
Correspondents say the questioning came across as a compelling psychological battle between the two former partners.
Muhammad, who said in opening statements that he and Malvo were innocent and "I'm going to prove it", pressed the younger man for details and questioned him about contradictory statements he has made.
On Tuesday, prosecutors questioned Malvo about their relationship, apparently aiming to portray Muhammad as a predatory figure who had brainwashed Malvo and turned him into a killer.
The prosecution team say they are pursuing the second trial in case the Virginia convictions are overturned on appeal.
They also say they want to serve justice in Montgomery County, where six of the 10 killings took place in October 2002.