Investigators have spent a year mulling every possible reason Noel Bankhead's SUV might have sped over a 50-foot cliff and plummeted into an Idaho reservoir on a Tuesday morning in June, killing the mother and her three children.

Maybe the red Land Rover had some sort of safety recall or mechanical failing? Perhaps Bankhead had a medical emergency - a seizure or stroke that caused her to lose control of the vehicle? Had she been impaired by alcohol or some medication she'd taken that morning?

One by one, investigators ruled out all of those reasons. Last week, they released their final conclusion: Bankhead purposely drove the red SUV into the water.

But instead of closing the case, the police conclusion elicited a new, bigger question for the Boise-area community mourning the sudden and tragic loss of a family:

Why?

Bankhead left no suicide note. And investigators say her mental state around the time of the murder-suicide remains a mystery.

But, as police described, her final actions were very clear:

“Witnesses... told investigators Noel Bankhead was driving her Land Rover northbound on Idaho 21, slowed down, turned onto Spring Shores Road, positioned the car toward the cliff, and suddenly accelerated,” the Ada County Sheriff's Office said on its blog. “Investigators found no skid or brake marks where the SUV went over the edge.

The crumpled hulk of the SUV sank 40 feet underwater. The occupants - Bankhead, her daughters Anika and Gwyneth Voermans, ages 13 and 8, and Bankhead's 11-year-old son Logan Voermans - died from a combination of blunt force trauma and drowning.

The investigators were never able to get information from the water-damaged event data recorder, which would show speed, braking, acceleration and any engine fault codes. Even the people at Land Rover's European offices were unable to recover the information.

Bankhead had gone through a divorce about two years before the crash, but there was little available publicly to show she was on the cusp of joining the sorority of mothers who've driven their children into bodies of water.

A study in the Journal of the American Academy of Psychiatry and the Law found that 80 percent of parents who had killed their children had ongoing mood or thought disorder, and 70 percent of mothers had previous contact with a psychiatrist or mental health professional.

In 2010, Shaquan Duley suffocated her two sons, then strapped them into their car seats and rolled them into a river. Family members said Duley suffered from severe depression, and had flown into a rage when she learned that her sister had given her sons a bath without her permission, according to ABC News. She was ultimately sentenced to 35 years in prison.

In 1994, Susan Smith, also of South Carolina, killed her two sons - three-year-old Michael and 14-month-old Alex - by strapping them into their car seats and pushing the car into a lake. She initially said her children had been taken by a carjacker. A jury declined to give Smith the death penalty, after learning about her history of physical and emotional abuse.

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Bankhead's ultimate motives may have died with her.

She was the oldest of eight children whose obituary described her as “a full-time working mom who loved to host family gatherings and wear beautiful dresses.”

“Noel loved her children, Anika, Logan, and Gwyneth,” the obituary said. “And she loved her large family.”