SANTA ROSA -- While offering sympathy to her family, Santa Rosa authorities said Thursday there was overwhelming evidence that Stanford student May Mengyao Zhou committed suicide by taking a massive dose of sleeping pills in the trunk of her car in January.

Zhou's father -- Yitong Zhou, of San Diego -- had come forward earlier this week and claimed an independent autopsy has revealed his daughter died of blunt force trauma to the head.

However, Santa Rosa Police Sgt. Paul Henry went through a litany of evidence that suggested the coroner's conclusions were correct including an email to her 16-year-old sister sent shortly before her January disappearance that a forensic physiologist had determined was "a good-by message."

"It offered life advice," Henry said of the message.

According to Henry, the toxicology report showed Zhou's body had a 6.15 mg/liter concentration of Diphenhydramine Hydrochloride -- the active ingredient of Unisom -- in her bloodstream.

The normal level for an adult, Henry said, was 0.10 to 1.0 while a dose from 1.1 to 5.0 was considered fatal.

During the police investigation, police discovered that Zhou purchased 32-capsule bottles of the medication at three different stores in the Menlo Park-Palo Alto area over a 35-minute period the night of Jan.18th.

Three empty vials of the medication were found in the trunk with her body.

Henry also said there were no signs of struggle in the trunk. She was fully clothed, there were no signs of robbery or visible trauma and a small sweater was used as a pillow under head.

"It appeared to be very peaceful," Henry said.

He added that police had requested copies of the private autopsy several times but had gotten no response from the family.

"We have no evidence to contradict our findings," he said. "We will examine any and all new evidence…Barring any new evidence we will conclude our investigation within a week."