A young Nova Scotia man suspected of killing two American sex offenders before taking his own life had visited an online registry that listed their names and addresses, a U.S. official said Monday.

Stephen McCausland, a spokesperson for Maine's Department of Public Safety, said the two victims were among 34 names that Stephen Marshall, a 20-year-old dishwasher from North Sydney, N.S., had looked up on the state's online sex offender registry.

Investigators searching for a link between Marshall and the murdered men discovered that Marshall had entered his name on the registry in order to get more information, including the street addresses of the victims.

Despite the possible break in the case, U.S. police are still struggling to understand why Marshall killed the two sex offenders before taking his own life.

"We don't have a link, we don't have a connection, and we have really more questions than we have answers as to what sparked this violence,'' McCausland said.

"At this point, there is no known connection between the three men.''

Marshall shot himself in the head with a .45-calibre handgun as officers boarded the bus on a ramp approaching Boston's South Station, said David Procopio, spokesperson for the Suffolk district attorney.

"Shortly after they stopped the bus and boarded the bus, they asked the driver to put the overhead dome lights on, which he complied with. A few seconds after the lights went on, Marshall shot himself with a .45-calibre handgun that he was carrying," Procopio told CTV Newsnet.

"There was no exchange of words that we're aware of. Witnesses didn't report hearing anything. Police officers did not report hearing anything."

Marshall was being sought as a person of interest in the shooting deaths of Joseph Gray, 57, and William Elliott, 24, whose bodies were found in towns 40 kilometres apart. Both men were registered sex offenders.

Moments after police boarded the bus, they heard a single gunshot and found Marshall with a massive head wound, Procopio said.

When paramedics arrived on the scene, they found a second handgun in Marshall's possession, Procopio said.

Marshall was rushed to Boston Medical Center where he was pronounced dead at 11:24 p.m., a hospital spokeswoman said.

No one else on the bus was injured, but five passengers were taken to hospitals after being splattered with blood.

Marshall would have faced firearms charges in Suffolk County and murder charges in Maine.

Investigators believe Marshall took three firearms from his father -- two handguns and a rifle. They have yet to locate the rifle.

Gray and Elliott were found shot to death in their central Maine homes, northwest of Bangor, early Sunday.

Police said Marshall had been driving his father's pickup, which was spotted leaving the shooting scene in Corinth at about 8:15 a.m., about five hours after the first shooting in Milo was reported.

The truck was later found abandoned near an arena in Bangor, Maine.

McCausland said Maine authorities told Boston police they believed Marshall was on a Boston-bound bus.

"That was because bullets had been found in a tank of a toilet in the men's room at the Bangor, Maine bus station and that someone matching Marshall's description had boarded the bus," he said.

"We alerted Boston police, they took it from there," he said.

The Maine Sex Offender Registry website lists the photos, names and addresses of more than 2,200 registered sex offenders.

The website was disabled while police searched for Marshall, but it has since been brought back up.

Gray's name was posted on the site because he had moved to Maine and had a Massachusetts conviction for sexual assault with a child under 14, McCausland said. Elliott's conviction was for having sex with a girl who was under the legal age, said McCausland.

Marshall, who lived in Cape Breton, Nova Scotia, had travelled to Houlton, Maine, to meet his father.

Police in Cape Breton said Marshall had moved out of his mother's home in Little Bras d'Or about a year ago. He was last seen in nearby North Sydney on Thursday and missed work on Saturday.

Const. Max Sehl told reporters that Marshall had no history of trouble in the community.

At the Chinese restaurant where Marshall worked, people remembered a "quiet kind of a fellow," in the words of co-worker Holly Tizzard.

"I am just like everyone else. I am in a state of shock. I don't believe it happened," said Charlie MacArthur, the restaurant's assistant manager who used to drive Marshall home.

He described Marshall as a good young man and a good worker.

With reports from CTV Halifax and The Canadian Press