The man convicted of killing a person in Virginia during the 2002 sniper killings has been found guilty of six more deaths in neighbouring Maryland.
The star witness was Muhammad's former protege
A jury took about four hours to find John Allen Muhammad, 45, guilty of all the murder charges against him.
Ten people were killed and three wounded - apparently at random - in the Washington area in October 2002.
Muhammad faces the death penalty for the conviction in Virginia. Maryland would jail him for life without parole.
Muhammad, who acted as his own lawyer during the latest trial, had argued he was framed. He sat with his arms folded across his chest as the verdict was read out.
He is due to be sentenced on Thursday.
Accomplice on stand
Muhammad's accomplice Lee Boyd Malvo, 21, was the star witness at the trial in Montgomery County, Maryland - the first time he has testified against his former mentor.
Malvo pleaded guilty and gave the prosecution extensive information about how he and Muhammad drove around in a car with a hole drilled in the back to allow them to shoot people.
Malvo was 17 at the time of the Washington killings
During the trial, Muhammad cross-examined Malvo. He was admonished several times by the judge for his line of questioning, at one point, calling Malvo "son".
"I would prefer you address me by my name," Malvo replied.
Muhammad apologised, only to do it again.
He 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.
"They said I was indoctrinated," said Malvo.
Correspondents say the questioning came across as a compelling psychological battle between the two former partners.
Prosecutors introduced DNA and ballistics evidence to link the two to the shootings which took place over three weeks in late 2002, terrifying people in and around the US capital.
The prosecution team say they pursued the second trial in case the Virginia convictions are overturned on appeal.
Muhammad suggested he had been framed for the killings
They chose not to seek the death penalty - a decision that has reportedly disappointed at least some relatives of the victims.
Muhammad and Malvo's Maryland victims were James Martin, 55 - the first Washington-area victim - James "Sonny" Buchanan, 39; Lori Ann Lewis-Rivera, 25; Premkumar Walekar, 54; Sarah Ramos, 34; and Conrad E Johnson, 35, the last.
Police as far away as Washington state and the Caribbean island of Antigua have said Muhammad may be linked to shootings in their jurisdictions.
He and Malvo could still face prosecution in connection with shootings in Alabama and Louisiana.