Although there has been no definitive link between the Web site and the murders, investigators have confirmed that Marshall had been on the state's sex offender Web site because the site requires users to register. In this case, Marshall registered with his real name and address, allowing police to see which profiles he had viewed.
However, Web site users could theoretically use a fake name and address.
While federal law requires each state to work within the requirements of community notification, some states have gone beyond the obligation by posting the personal information of sex offenders online.
"It's easier to meet the requirements by providing information on the Web site," said Charles Onley, research associate for the Center for Sex Offender Management, an office of the Department of Justice. "But there is no [federal] requirement of Internet access."
The states' Web sites are not always comprehensive. It is within the individual state's jurisdiction to determine which sex offenders pose a higher risk to the community, and it can make their information more accessible than others'.
Onley said some states, like Pennsylvania, have been more conservative about the information they provide the public. Other states, like Florida, have been more liberal in their approach.
"There has been a drift toward full disclosure [of personal information]. The thought was to go out and inform the community," said Onley, who explained that there was a lot of public hostility toward sex offenders when federal law required that their whereabouts be made available to the public.
"The thing is, you have the mandate and the public wants the information. The information might be less valuable [without addresses]," said Onley. "The public would want addresses."
All states do not require that sex offenders' addresses be made public, though many do provide that kind of detail.
The National Sex Offender Registry provides online information on sex offenders in each state except Oregon and South Dakota.
"It's not just that it is online, [anybody] could have gone down to the sheriffs office to get the information," said Onley.
While incidents of vigilante violence against sex offenders are rare, this week's case is not the first.
In September Michael A. Mullen of Bellingham, Wash., was charged with the murders of two convicted sex offenders after he admitted to police that he wanted to kill them after he saw the information listed on the state's online sex offender Web site. He sits in jail awaiting trial.
In a 2004 case, the state of New Hampshire sent 57-year-old Lawrence Trant to jail for 10 to 30 years after he pleaded guilty to charges of attempted murder on two convicted sex offenders whose names and addresses he'd located on the state's sex offender registry.
In an interview with The Boston Globe, Trant said, "I don't want people to steal the souls of little kids. I'm doing 30 years for something I think is morally justified."
Maine investigators have yet to examine a laptop computer Marshall carried with him on the bus before he committed suicide.
Investigators hope it can explain why Marshall was apparently driven to kill the two men from Maine.
The Associated Press contributed to this report.