Twenty years later, one of Arizona's most infamous cases still brings in tips, while leaving friends and investigators split on what happened to the man at the center of the mystery.

Robert Fisher, who would turn 60 this month, is accused of murdering his wife and two children and blowing up their Scottsdale home in April 2001. He was never found and remains a mainstay on the FBI's Ten Most Wanted list.

"This is, by far, the most heinous crime that we've had here in Scottsdale," said Scottsdale Police Detective John Heinzelman.

Heinzelman has been assigned to the Robert Fisher case for the past five years, and said tips still come in to this day.

"We still get tips on a weekly basis, if not daily basis," Heinzelman said. "The tips we get vary from any number of things, it could be something as simple as, 'I was looking at Facebook and I saw a person named Bob or Robert Fisher.'"

What happened to Fisher remains a mystery. His Scottsdale home exploded on April 10, 2001. Investigators later found the bodies of his wife, Mary, and two children, Brittney, 12, and Bobby, 10 inside the home.

Editor's note: This story contains details about the deaths of Mary, Bobby, and Brittney Fisher that some viewers might consider disturbing. Reader discretion is advised.

The case and the big question: Is Robert Fisher still alive?

"They died brutally," Heinzelman said. "Mary was shot and her throat was cut, and both Bobby and Brittney had their throats slashed, almost from ear to ear."

Authorities determined a gas line to the home had been severed and the house rigged to explode.

Robert Fisher was not found at the scene, but was spotted on an ATM camera withdrawing $280 the night before the fire.

"I think blowing up the house was purely a delay tactic, a discovery tactic. He knew that the bodies were going to be found," Heinzelman said. "Maybe in his thought, if the entire house is incinerated, maybe it's believed that he perished in the fire along with his family. Maybe it's purely an evidence eradication."

Mary's 4Runner was found 10 days later in the woods near Payson. Fisher was nowhere to be found. Heinzelman noted that the area is extremely remote and has a lot of caves.

Additionally, the family dog, 'Blue,' was located near the vehicle.

"That's been the topic of most conversation, is why was the dog still there?" said Heinzelman. "If he committed suicide, if he walked off into the woods, the dog would have been there with him. So clearly, he must have gotten into a vehicle and left, and the dog just was abandoned behind. So that's that one piece that I really wish we would have been able to kind of make heads or tails of..."

"The other part to that is, it is less than a mile away from the Apache Indian Reservation," he said. "That is a sovereign nation that we don't travel onto."

According to the Scottsdale police report, Fisher had previously enlisted in the U.S. Navy but failed in his attempt to become a Navy SEAL and was described as an outdoorsman.

Heinzelman told ABC15 Fisher is the lone suspect in the murders.

Different detectives have different theories, but they continue to keep the investigation open, and look into every tip that comes their way.

While no official motive for the murders, friends and investigators have described Mary and Robert's marriage as one that had problems, and Fisher had taken issue with his own parents' divorce. That, according to police, is one theory.

"The fear that we believe he had of being divorced was very strong," Heinzelman said. "He's in despair, he's in his lowest moment, he's afraid that Mary's going to divorce him and he's going to lose his children."

A Scottsdale police report spans more than 400 pages and details odd behavior from Fisher throughout his life, in interviews with friends and coworkers. According to the report, Fisher would randomly shoot his gun in the air when on a hunting trip with friends and was also seen in a photograph covered in animal blood standing next to a deer he killed.

"As an investigator, we want all the puzzle pieces, we want to see what the picture looks like at the end," Heinzelman said. "Right now, there's a gap, we're missing some pieces."

Read more about the investigation and what questions remain unanswered by clicking here.

Who is Robert Fisher?

Robert Fisher would turn 60 years old in April. He was never found after his Scottsdale home exploded on April 10, 2001, with his wife, Mary, and two children, Brittney and Bobby, inside. Investigators later determined his wife and children were murdered and the home was rigged to explode.

A police document that spans more than 400 pages details a rocky marriage between Robert and Mary.

Their former Scottsdale pastor told ABC15 he was helping the couple deal with their issues.

"Mary was sweet," said Pastor Gregg Cantelmo. "He would probably say she was a little difficult at home... everybody has problems. They didn't have any unique problem that was different than anybody else in the church."

According to interviews compiled in the Scottsdale police report, Fisher had admitted to an affair roughly two years before the fire. Fisher was kicked out of the house, but he and Mary ultimately reconciled and continued their rocky relationship, according to the report.

"That was the conversation, in the midst of some marital problems, we'd talk about how to get through those," Cantelmo said. "How to forgive one another and how to move forward."

Robert Fisher's own parents divorced when he was a teenager, and investigators have pointed to his aversion for divorce as a potential motive.

According to the police report, Fisher was an avid outdoorsman. He previously worked as a firefighter and was working as a respiratory therapist at a Valley medical center when the fire occurred.

"He was a quiet guy," Cantelmo said. "I wouldn't say there was any one emotion that really presented itself other than he was just quiet. He was nice but he was quiet."

The lengthy Scottsdale police report details odd behavior from Fisher throughout his life.

"Those two things can coexist, you can have somebody who's struggling with some mental health issues but who also has capacity to plan and cover up tracks and whatnot," said Dr. Tess Neal, a forensic psychologist and assistant professor of psychology at Arizona State University.

Neal has not worked on the Fisher case, nor interviewed anyone involved, but ABC15 asked her to weigh in on details that are publicly available, including the police report.

"If there were indications that this marriage may have been coming to some kind of end, and it looks like maybe there were some of those indications, that he really just did not want that to happen," Neal said. "[He] did not want his children to have to live through the kind of divorce-related trauma that he himself had experienced or his subjective interpretation of what that experience was like."

Read more about Fisher's background and expert analysis on his behavior by clicking here.

What do police believe Robert Fisher looks like now?

Despite the lack of clues, investigators have not given up on their mission to find Fisher.

Those who knew the couple say they were having marital problems, and Fisher may have lost control after learning his wife Mary planned to divorce him. Police say Fisher slashed the throats of his wife and two children, Bobby and Brittney, before rigging up his home's natural gas line to explode, then disappearing.

Police believed the fire may have been an effort to conceal the horrific crime and delay police as he went on the run.

The last clue Fisher left behind was Mary's car, dumped in the Tonto National Forest near Young, Arizona. Police say the family dog was found alive by the car. There was no sign of Robert Fisher.

Despite a massive search in the area, Fisher has remained missing to this day. Police said they still get tips in this case and look into every single one of them.

If alive, police say Fisher looks different from the original photos of him depicted in family pictures and home videos.

"You have to remember everybody is 20 years older. So Robert Fisher will turn 60 in April. That is the first thing," said Detective Heinzelman.

"We're not looking for a 40-year-old person anymore. We're looking for a 60-year-old person. Now, maybe his height won't change, but his weight certainly could change, his hair could change, his facial features."

The FBI used special aging software to recreate multiple different looks Fisher could have adopted as he grew older.

Read more about what Robert Fisher may look like now by clicking here.