The mysterious death of Plano middle school teacher Libby Davis has flummoxed Collin County officials, even after an autopsy and an investigation.

In July, the 28-year-old drove off the Bush Turnpike, crashed into a grove of trees and then ran from her car. Davis was found dead the next day, floating face-down in 2 to 3 feet of water in a Plano creek.

Davis' death led to an outpouring of support for the young mother's family online and raised the specter of postpartum depression's dangers — even though it's unclear what role, if any, the mood disorder played in her death.

The Collin County medical examiner's office said Tuesday that Davis' cause of death is "undetermined." And it might forever be.

It's possible she died from a blunt-force injury to her head, by drowning or heat-related illness, Medical Examiner William Rohr said in his report. Police never suspected foul play.

Libby Davis memorial video:

The autopsy report showed Davis had scrapes and bruises on the left side of her face, including around her eye, nose and ear. Davis also had two prescription drugs in her system: sertraline, the generic form of the anti-depressant Zoloft, and lamotrigine, which is primarily used for treating seizures and bipolar disorder.

Davis had suffered from postpartum depression after the birth of her daughter, Michelle, who was 20 months old when her mother died.

Davis was a teacher at Hendrick Middle School in Plano. Her Facebook bio describes her as "Libby = Friend. Daughter. Sister. Special Needs Teacher. Wife. Momma. Missionary Kid. Texan."

According to Facebook, she "grew up drinking coconuts and swimming in ocean as a missionary kid in the Solomon Islands. She attended high school in the highlands of Papua New Guinea."

Family photos on Facebook speak to happier times. In one picture, the family poses with Michelle not long after her birth. Michelle wore a pink bow and may have been yawning. Davis' husband, Kevin, holds Michelle and grins proudly. Libby Davis smiles and looks happy.

Other photos show Libby holding Michelle in the snow, on her first birthday and up in the air as Michelle's little hands squeeze Libby's face.

"Love her to pieces," Libby Davis wrote on Facebook. "

Also, super thankful for my amazing sister in law for always being our photographer."

Libby Davis memorial service:

Davis' husband could not be reached for comment. But he paid tribute to her on Facebook after she died.

"It's with an extremely heavy and broken heart to say my beautiful wife of 7.5 years, my rock, and Michelle's 'mama' has passed away from this life. Libby was such a bright light in this world, but if you knew her, then of course my telling you is completely superfluous," Kevin Davis wrote on Facebook.

"Words can't express the loss this is for me and our baby; that Michelle will grow up without her sweet loving mother is a fact that I can hardly stomach. ... I feel so lucky to have been married to this stunning angel of a human being for the time I had."

Postpartum vs. "baby blues"

Postpartum depression is more intense and lasts longer than "baby blues," a term used to describe the worry, sadness and tiredness many women experience after having a baby, according to the Centers for Disease Control and Prevention.

More severe is postpartum psychosis. It is a rare illness, occurring in less than 0.2 percent of births, according to Postpartum Support International, and it usually appears within the first two weeks after delivery.

Women with postpartum psychosis can have auditory and visual hallucinations. They can hear voices that tell them to harm their children or themselves, experts say.

Research cited by the postpartum health advocacy group indicates there is a 5 percent suicide rate and a 4 percent infanticide rate with postpartum psychosis.

Where can I get help if I suffer from postpartum depression, baby blues or postpartum psychosis?

See your doctor or call the Postpartum Support International helpline at 1-800-944-4773.