At the time of her death, 23-year-old graduate student Mengyao “May” Zhou had toxic levels of an over-the-counter drug in her body, investigators said Friday.

The drug, diphenhydramine, is most commonly found in sleep aids and in Benadryl, an antihistamine medication used to minimize allergic reactions. According to the Sonoma County coroner, whose office conducted Zhou’s autopsy, “potentially toxic” levels of diphenhydramine can be anywhere from 1-5 mg/L.

Zhou had 6.15 mg/L in her body at the time of her death, toxicology tests showed.

"No other common acidic, neutral or basic drugs [were] detected," a press release from the Sonoma County Sheriff’s Office said. "No blood or urine ethyl alcohol [was] detected."

Sergeant Mitch Mana of the Sonoma County Coroner’s Office said that he could not elaborate further on the information provided in the press release.

An official cause or manner of death has yet to be determined, and the case is still under investigation. Messages left with Santa Rosa Police were unreturned by press time.

Zhou, a second-year doctoral student in Electrical Engineering, was found in the trunk of her car on Jan. 25, four days after being reported missing by her roommate. Authorities have called her death an apparent suicide.