This is the first story in a new weekly series that profiles America's most wanted criminals.
To neighbors, Robert William Fisher seemed like an average guy.
He loved to hunt and fish. He doted on his kids and dedicated himself to a career of helping others, first as a firefighter, then later as a surgical catheter technician and respiratory therapist.
But there was a dark side to Fisher. People close to him described him as a loner, a control freak who didn't want anyone outside of his close-knit family raising his kids.
It could have something to do with his parent's divorce, when he was just a teenager, investigators say. He went to live with his dad, but when his father remarried, Fisher took it hard and vowed to never divorce.
On April 10, 2001, police suspect that dark side got the better of him.
He and his wife Mary, who were having marital troubles, were having an argument at their Scottsdale, Ariz. home. The night before, neighbors heard yelling from the house.
"We don't know what it was about," said FBI special agent Robert Caldwell. "Mary was probably getting ready to leave him. Rather than have someone else raise his kids, it was better off to start off a clean slate."
Also at home that night were Brittney, 12, and Bobby, 10.
Caldwell said Mary, who had already caught Robert cheating with another woman, learned he was having another affair. Mary already took Robert back one time. She wasn't going to do it again and planned to leave him.
She would never get the chance.
Investigators say Robert shot Mary, 38, in the head, then hunted down his two children and slit their throats. Fisher is thought to have then put an accelerant on their bodies, opened a gas line to the heater and lit a candle down the hallway that caused the house to explode.
That night, Fisher just vanished without a trace, Caldwell said.
"This guy, he just cut all ties with everyone," Caldwell said. "He just disappeared."
• Click here for photos of Fisher.
Nearly seven years later, Fisher remains the only suspect. He is wanted for unlawful flight to avoid prosecution, three counts of first-degree murder and arson of an occupied structure.
He does not have a prior criminal record and no history of civil disputes.
Authorities have few clues on where he might be, and say because of his anti-social lifestyle, he's unlikely to make friends or share grisly secrets about his past.
"He's perfectly fine to be on his own," Caldwell said, who became involved in the case in February 2002. "If you keep your identity low and you're not showing your face or contacting friends and family, it's easy for you to avoid detection."
Thousands of tips have poured in from across the country but have led to dead ends.
A few years ago, authorities thought they had found their man after a sighting in Canada. The tipster said Fisher's description matched that of the tipster's mom's new boyfriend.
Canadian authorities conducted surveillance and captured the man. But the fingerprints didn't match Fisher.
Fisher remains at large, and is considered armed and dangerous. Authorities want him dead or alive.
"We have to assume he is alive until we find his body or him," Caldwell said.
Authorities believe Fisher may have killed himself, but the search continues.
"We definitely want to see closure for the family and locate and arrest Robert," Caldwell said.
The 6-foot, 190-pound fugitive, has a gold crown on his upper left bicuspid tooth. He sometimes walks with an exaggerated erect posture because of back problems. He also chews tobacco.
The FBI is offering a $100,000 reward for information leading to Fisher's arrest.