McLEAN, Va. – It galled her to do it, but Sarah Dillon was desperate for answers, so she wrote letters to convicted snipers John Allen Muhammad and Lee Boyd Malvo: If you murdered my son, please confess, she wrote.

She got no reply.

“I’ve been waiting for answers for seven years,” said Dillon, who took to wearing a button that said “Billy Gene Dillon is a very important person” as a reminder that his killing remains unsolved.

Sarah Dillon is not the only person with unanswered questions about the killing spree initiated by Muhammad and Malvo seven years ago, which culminated with 13 shootings and 10 deaths over a three-week span that terrorized the Washington region.

As Virginia prepares to execute Muhammad on Tuesday, authorities are unable to answer perhaps the most basic question about the killings: How many people did he and Malvo shoot and kill?

The killing spree in the Washington area in October 2002 is well documented. Beginning on Oct. 2, Muhammad and Malvo shot 13 people at random with a high-powered rifle, firing from the trunk of a modified, beat-up Chevy Caprice. Ten were killed before authorities finally tracked down the pair at a Maryland rest stop.

But the sniper shootings started before Muhammad and Malvo reached the Beltway, with victims killed or wounded as the duo drove across the country.

Malvo and Muhammad lived in Tacoma before heading east and beginning their string of random sniper shootings. Malvo has admitted killing Keenya Cook in Tacoma in February 2002, telling police and psychiatrists that the shooting was Muhammad’s way of testing him

Investigators have clearly linked the two to some other prelude shootings, though they have never stood trial for them. Others fall into a gray area – police have suspicions, perhaps, but no proof.

The question became even murkier in 2006, when Malvo reportedly confessed to four additional shootings, including two killings, that had not been linked to him.

If Malvo’s reported confessions are accepted as true, it would mean he and Muhammad are responsible for 27 shootings resulting in 17 deaths in 10 states (Maryland, Virginia, Alabama, Washington, Georgia, Texas, California, Florida, Arizona and Louisiana) plus the District of Columbia.

But Malvo would talk to police only in jurisdictions that promised not to prosecute him, a deal some agencies weren’t willing to make.

So in Clearwater, Fla., the golf course shooting of Albert Michalczyk on May 18, 2002, officially remains unsolved, though Michalczyk took Malvo’s reported confession in 2006 as confirmation of something he long suspected.

“My wife immediately thought it was these guys,” Michalczyk said at the time. “We put two and two together, but we never came up with four. Now, we are coming up with four.”

Police from Tucson, Ariz., consider the golf course killing of Jerry Taylor solved, based on their interview with Malvo, which they obtained only after agreeing not to prosecute him.

The victim’s daughter, Cheryll Witz, decided that knowing the truth was more important than seeing Malvo face criminal charges, given the fact that he was already serving life in prison. At one point, Malvo even called Witz on the telephone and apologized.

Back in Texas though, Sarah Dillon still doesn’t know who shot and killed her son, Billy Gene Dillon, 37, in May 2002 outside a rural Denton County home about 40 miles north of Dallas. Local authorities submitted bullet fragments in 2002 from their investigation of Dillon’s death to the task force that investigated the sniper shootings, but tests were inconclusive.

At the time, they had little reason to suspect the snipers except for the fact that Dillon had apparently been shot at a distance by a high-powered rifle, just as the victims of the D.C. sniper spree were. Police agencies from across the country took similar actions, to see if unsolved killings could be connected to Muhammad and Malvo.

Denton County sheriff’s spokesman Tom Reedy expressed some frustration about the inability to get answers from the Washington-area authorities regarding Billy Gene Dillon’s death.

“If they give you an answer, let us know,” he said.

The FBI, part of the sniper task force that helped eventually catch Muhammad and Malvo, declined to comment on how many people the snipers shot and killed, except to say the question is “complicated.”

“To further complicate it, the statements of Muhammad and Malvo need to be relied on as to who performed any given shooting. Needless to say, their statements cannot be vetted for each and every event,” FBI spokesman Richard Wolf said in an e-mail.

State and local authorities, including Fairfax County Police, Montgomery County Police, the Montgomery County State’s Attorney and Maryland Office of the Attorney General all referred the question to other agencies.