The jury's decision came after a drama-filled trial in which the defendant, John Allen Muhammad, acting as his own attorney, grilled his young accomplice, Lee Boyd Malvo, on the witness stand.
Muhammad has already been sentenced to death for one of the murders in neighbouring Virginia, where Malvo is serving a life sentence.
In total, the three-week killing spree claimed 10 lives in Maryland, Virginia and Washington, and spooked a region still living in dread of a repeat of the September 11 attacks and deadly anthrax mailings a year earlier.
Malvo, 21, provided dramatic courtroom testimony last week as he faced off with his former mentor, calling Muhammad, 45, a "coward" and accusing him of turning him into a "monster."
The young murder convict told how he and Muhammad mounted a three-week rampage against ordinary people picked off in the sprawling US capital region as they pumped petrol, waited for a bus, or shopped.
Malvo testified that Muhammad had told him "There are going to be six shootings a day for 30 days" targeting the largely white, affluent suburbs around Washington, a city overwhelmingly populated by African-Americans.
The first month of killings was to have been followed by bombings and attacks on schools, Malvo said.
"For the sheer terror of it, the worst thing you can do to people is aim at their children," Malvo quoted Muhammad as saying.
Malvo said Muhammad planned to use the spree to extort 10 million dollars to set up a training camp in Canada, to send out 140 young men to wreak terror all over the United States.