Defense theory: Shooter used her toe to pull the trigger

On July 14, 2017, Fairfax County, Virginia, patrol officers busted through the front door of a million-dollar home. They quickly discovered the bodies of Pam Hargan, 63, and her daughter, Helen, 24, who had a rifle at the scene. Almost as quickly, police told reporters it was a murder-suicide and told Helen's father her wound appeared to be "self-inflicted."

But when lead homicide Detective Brian Byerson looked at the evidence, he wasn't so sure.

So, he made a decision: "Wait. Wait and do the job," Byerson tells "48 Hours" correspondent Peter Van Sant in "Death Hits Home: The Hargan Killings" airing Saturday, October 1 at  10/9c on CBS and streaming on Paramount+. "You will make mistakes in these cases if you make assumptions before you actually do the work."

Though investigators found no suicide note, Helen's oldest sister, Megan, 34, told them Helen was "depressed" and had been "so angry, like, just so angry all the time."

Megan had been living at home with Pam and Helen. She told investigators her mom and sister had been fighting, but that Pam was alive when Megan left the house that afternoon.

According to Megan, Pam announced that very morning that she was canceling the contract on the house she was buying for Helen. Why? Because Pam did not approve of Helen's boyfriend who was planning to move in.

Soon thereafter, Pam was dead.

Could this have been the motive for Helen to kill her mom and then take her own life?

As investigators combed the house for clues, the medical examiner – aptly named Dr. Posthumus – delivered Helen's autopsy findings: She had been shot in the top of her head. The rifle bullet had traveled downward into her neck.

Byerson says it's impossible for Helen to have shot herself in that way with that gun.

"She would have to hold it straight up and be able to reach the trigger to accomplish this," he says. "She would [have] had to have done magic. It just isn't possible."

As far as Byerson was concerned, this was no murder-suicide. This was a double murder.

But who was the murderer?

"Our job is to try to put that puzzle together and figure out what happened," says Byerson.

As he and his squad continued to investigate the killings, Byerson says that the evidence did, in fact, point toward one of Pam Hargan's daughters. But it wasn't Helen. 

"It becomes very obvious to us," he says. "It's Megan Hargan."

Byerson soon learned that Megan had a six-figure motive for wanting to kill her mother. Pam Hargan was a wealthy woman with an $8 million estate, and Megan Hargan wanted a house of her own.

Five days after Pam and Helen's deaths, Byerson brought Megan in for an interview. It would last for more than four hours. He says she did confess to trying to steal her mother's money, but she was adamant that she hadn't killed her family.

Despite Byerson's lingering suspicions — and a growing pile of evidence against Megan Hargan — police did not arrest her that day.

That would take almost a year-and-a-half — until November 9, 2018.

"Why in the world did this take 16 months to file murder charges?" asks Peter Van Sant.

"So, murder investigations can be extremely complex," says Det. Byerson. "You not only have to be sure, you have to be right. And that decision … does not just rest on me. … I have to be on the same page as the Commonwealth Attorney's Office. So, in consultation with them, we decided to wait."

The surviving Hargan family would have to wait three-and-a-half more years for Megan Hargan to stand trial.

Commonwealth of Virginia prosecutors Tyler Bezilla and Whitney Gregory opened by telling the jury that on the day before and the morning of her mother's murder, Megan had tried to secretly steal more than $400,000 for a new house from Pam's bank account. Pretending to be Pam, Megan called her mom's bank and tried to wire transfer the money.

The prosecution says Megan got so desperate for the cash that she killed her mother. Then, the theory goes, Megan killed her sister to keep her quiet, staging the scene as a murder-suicide.

"This is an individual who murdered two of her closest family members for money," says Bezilla.

But the defense insists Helen – not Megan – was the killer. Megan's lawyers describe Helen as mentally unstable and depressed. They tell the jury that Helen was furious at her mother for saying that if Helen didn't break up with the man she hoped one day to marry, she wouldn't get the new house.

The prosecution contends there is no evidence that Helen was depressed.

But the defense argues police got it right the first time. This was a murder-suicide.

Forensic specialist Iris Dalley Graff reconstructed Helen Hargan's death scene. She testified on behalf of the prosecution and told Peter Van Sant that Helen's arm was not long enough to pull the trigger.

The defense has a surprising theory, one that doesn't rely on the length of Helen's arm.

They tell the jury Helen Hargan shot herself in the head with the rifle by pulling the trigger with her toe.

Strange? Improbable? Perhaps.

But as Graff conceded to Van Sant, though highly unlikely, it is possible. "Her legs are long enough that her toe could reach the trigger," she says.

In their closing argument, prosecutor Bezilla challenged the defense's assertion that Pam Hargan was going to cancel the contract on Helen's new house. But the defense insists there's a mountain of reasonable doubt in this case. They argue the prosecution's forensics are inconclusive and the "toe on the trigger" theory cannot be ruled out.

Will the jury believe it?

In:
• Helen Hargan
• Megan Hargan
• Pam Hargan
• Murder