It was early autumn in 1969 when a fire ripped through a Massachusetts home in the middle of the night, killing a young mother and five of her children in a tragedy that was ruled accidental.
Now, 43 years later, a family member's dogged research and the startling reappearance of an ex-con at the family grave site on the anniversary of their deaths has authorities questioning whether the blaze was an accident or murder -- and eyeing the man as a person of interest.
"I would say it's really doubtful that this was any kind of electrical fire," Wilmington Police Chief Ed Bradbury said in an interview with FoxNews.com.
"My gut tells me this person had something to do with it," said 58-year-old Bradbury, who in 1969 was a teenager living down the street from the Landers' home in Wilmington, about 10 miles north of Boston.
"Unfortunately, the scene is gone and there are no forensics that remain," he said.
Bradbury and police detectives reopened the case in 2010, after the wife of a surviving family member approached them with research of her own. Janis Jaquith, who married Harry Landers, the oldest son, had conducted extensive interviews with neighbors and firefighters who responded to the scene that night. Her findings convinced the family that the tragedy long ago was no accident.
"There were some basic police reports, but no investigative files from 1969," retired Wilmington Police Lt. Chris Neville said. "There was nothing that really examined the cause."
At approximately 3 a.m. on Sept. 26, 1969, David and Nancy Landers awakened to what they thought was an earthquake, according to Bradbury.
"The house shook," he said. "The mother and father got up and saw fire outside of their bedroom window."
David Landers, three of his children and a cousin managed to escape the inferno that spread through the two-story house within minutes.
His 37-year-old wife and five other children -- Davey, 13, Billy, 12, Kevin, 9, Lisa, 7, and K.C., 4 – perished.
Fire officials were quick to label the fire "electrical," speculating that the blaze likely began with a faulty power outlet behind a sofa in the first-floor living room.
But old black-and-white photos of the scene, when examined by arson investigators, suggest two points of ignition. And the blaze was not characteristic of an electrical fire, Bradbury said.
"Electrical fires tend to smolder for a long time," he said. "You have a wire that heats up. It doesn’t begin as this intense fire…It’s going to start small and build from there."
"That was not the case here. This house should have been full of smoke, but it wasn’t," Bradbury said, adding that the explosion-like sensation the Landers felt points to an accelerant.
Authorities said it was common at the time to rule fires "electrical" when the cause was unknown.
"That’s how it was back then. There wasn’t a whole lot of science. They couldn’t test for accelerants. When they couldn’t determine it, it was electrical," he said.
A strange twist in the case came a few months later.
Mr. Landers and his surviving children lived for several months in a trailer on their property while their house was being rebuilt.
According to police and family members, Landers reported smelling gasoline one night outside the trailer. When the father stepped outside to investigate, he found burning newspapers, soaked in gasoline, placed underneath the trailer. He quickly stomped out the fire and notified police, who dismissed the incident as a prank, unrelated to the fire that killed his family.
"'A tragedy like this will bring out the crazies,'" police told Landers, who died in 2010.
Law enforcement sources told FoxNews.com that the person of interest, who is not being publicly named, lived near the Landers' home and was a teenage acquaintance of then 17-year-old Harry Landers.
His sister, Susan Landers, who was then a 15-year-old freshman at Wilmington High School, survived the fire after her father placed her onto the roof through a second-story window.
In an interview with FoxNews.com, Landers, whose married name is McNamara, said she was pulled from a classroom in the days that followed by two police officers, who asked her about what relationship, if any, she had with the person of interest. Nothing further came of it, McNamara said, and police records from the time were either lost or destroyed.
"There were a lot of dots that should have been connected that weren’t," McNamara said.
Over the years, members of the Landers family said they spotted the person of interest near their home on multiple occasions on the anniversary of the fire.
McNamara recalled a chilling encounter she had with the man last May. She said the man, who is now 62, approached her while she was visiting her family's grave site on Memorial Day.
"This man has popped up 43 years later at my family’s gravestone, while I was there," McNamara said. "This person has no business being in our lives whatsoever."
"He said a whole series of things to me, including that he was a Christian man now," she continued. "He claimed he spoke to my father before he passed away and that my father gave him forgiveness."
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Police said the man in question has a long criminal history, including a 1994 charge of "armed assault with intent to rob" and a 1987 conviction of "attempted robbery in the third degree."
Neville said the man has declined through an attorney to speak with investigators.
For the Landers family, the anguish of losing six family members was so unbearable that McNamara said she and her siblings never spoke about the fire for 40 years. Community members as well, including the fire department, hardly uttered a word over the years about the tragedy.
"The pain was of such magnitude that the best way to go on with your life was to not talk about it," McNamara said.
But more than 40 years later, the Landers family hopes that those with knowledge of what happened that night will talk about it.
"There’s a big difference between an accidental tragedy and mass murder," McNamara said. "If it’s the latter, my family needs justice and I’ll do whatever I can to get it."
Cristina Corbin is a Fox News reporter based in New York. Follow her on Twitter @CristinaCorbin.