John Allen Muhammad, pictured in 2004.

Jeanne Meserve and Mike M. Ahlers
CNN

John Allen Muhammad, the mastermind behind the Washington-area sniper attacks of 2002, is scheduled to die by lethal injection Tuesday evening at a state prison near Jarratt, Va.

During two lengthy trials - including one featuring testimony from young accomplice Lee Boyd Malvo - and in several years of legal appeals, Muhammad has continued to profess his innocence.

On Monday, the Supreme Court denied an appeal, meaning Muhammad, 48, is likely to go to the death chamber at Greensville Correctional Center at 9 p.m. Tuesday.

The only remaining obstacle is a clemency petition to Virginia Gov. Tim Kaine, who has already indicated he is not apt to grant clemency. Lawyers for Muhammad also could file additional appeals, although it is not clear on what grounds.

If Muhammad enters the death chamber without acknowleging his crimes, he will likely be known as the progenitor of one of the most enigmatic mass murder teams in history: Muhammad - a Gulf War veteran who was described as a "gentle man" by acquaintances; and Malvo, a young Jamaican youth on a desperate search for a father figure who had gotten an "A" in a Bellingham (WA.) High School citizenship class shortly before the killings began.

Prosecutors say Muhammad, fueled by grudges against the Army and his ex-wife, plotted the vicious cross-country shooting rampage, culminating in a killing spree in Washington, DC, which cloaked the region in fear and which captivated the nation's attention.

During three weeks in October 2002, Muhammad and Malvo killed 10 people and wounded three, while taunting police with written messages and phoned-in threats and demands.

The rampage could have been worse.

During Muhammad's second trial in Maryland, Malvo testified that Muhammad originally planned to kill up to six people each day for 30 days.

Indeed, after killing five people during the first 24 hours, the sniper team began the second day scouting for locations for another barrage of shootings. But, concerned about possible witnesses and the lack of escape routes, they slowed their pace of killing.

Prosecutors say Muhammad intended the killings to provide a smokescreen to cover up his real goal; he hoped to kill his wife Mildred and gain custody of his three children.

Defense attorneys and some supporters say Muhammad is mentally ill, and suffered post traumatic stress disorder (PTSD) during his service in the first Gulf War. Defense attorneys have released a scan of Muhammad's brain which they say shows signs of brain damage to both the front and rear of his brain, consistent with schizophrenia and other brain dysfunction.

Though Muhammad worked with explosives in the Army, his weapon of choice for the killing spree was a Bushmaster XM-15 rifle and .223 ammunition, usually fired from close range from the trunk of an old Chevy Caprice. Muhammad cut a hole in the back of the car, and removed a wall between the trunk and the back seat, turning the car into a sniper's nest.

But several of the shots were fired outside the vehicle, including the shot that wounded 13-year-old student Iran Brown, the youngest of the victims, and bus driver Conrad Johnson, 35, the last victim.

The investigation was marked by a high-level of involvement from local, state and federal law enforcement authorities, who flooded the Washington area. But it was also marked by missed opportunities.

An early focus on white box trucks - one was seen speeding near the scene of one shooting - meant reports of a Chevy Caprice were ignored or discounted. Another missed opportunity came when investigators found a note in a baggie tacked to the tree of a shooting site. The note instructed police to answer a nearby phone at a certain time. But by the time police processed the baggie for fingerprints and DNA, the appointed time had passed.

In notes left at shooting scenes, the snipers demanded $10 million to stop the shootings.

The big break came when an Army friend of Muhammad's called authorities to voice suspicions about Muhammad.

Muhammad and Malvo were captured at a Maryland rest stop, but were transferred to Virginia because of the state's aggressive use of the death penalty. Ultimately, Muhammad was convicted of capital murder and terrorism charges for killing Dean Harold Meyers, a Vietnam veteran cut down by a single bullet that hit him in the head October 9, 2002, as he filled his tank at a Manassas, Virginia, service station.

Muhammad also stood trial in Maryland, and was convicted of six murders there.

Malvo was tried in Virginia for the Oct. 14, 2002 murder of FBI analyst Linda Franklin, 47, outside a Home Depot in Fairfax County, Va. A jury sentenced Malvo to life in prison after defense attorneys said Malvo, who was 17 at the time of the murders, was brainwashed by Muhammad.

Malvo testified against Muhammad at Muhammad's Maryland trial, calling him a "coward."

"You took me in your house and you made me a monster," Malvo testified. Malvo, now 24, is serving time in a Virginia prison.

Malvo said the Washington killings were "phase one" of a three phase plan. In subsequent phases, Malvo said, Muhammad planned to use $10 million to create a utopian community for homeless people in Canada, at which young people would be trained to go launch additional attacks in the United States.

But whether that was Muhammad's real intent is still a matter of conjecture.

Some acquaintances believe that after Muhammad recovered his children, he planned on killing the one witness who could provide the most threatening testimony against him - Lee Boyd Malvo.