Vancouver university student Elisa Lam, whose mysterious death during a stay at a Los Angeles hotel sparked speculation from amateur sleuths around the world, accidentally drowned, according to the L.A. coroner's office.

The long-awaited finding, four months after Ms. Lam's body was discovered in a water tank on top of the Cecil Hotel, noted that the 21-year-old suffered from bipolar disorder.

Lieutenant Fred Corral of the coroner's investigation branch said Thursday that would likely explain Ms. Lam's strange behaviour in an elevator on the last night she was seen in public.

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A closed-circuit video showed her pushing numerous buttons, hiding in a corner, peeking out from the elevator, pressing more buttons, and finally waving her arms about before walking away, with the elevator still on the same floor.

After it was posted by police seeking clues to Ms. Lam's disappearance, the video attracted hundreds of thousands of Internet hits and a raft of speculation on what might have prompted her actions.

The discovery of her body a few weeks later, wedged near the bottom of the hotel's rooftop water tank, which was not easy to reach, only added to the mystery.

Asked how Ms. Lam might have made it up to the tank, Lt. Corral replied: "I guess she climbed up there."

How she could have drowned was not covered by the coroner's conclusion.

Toxicology tests disclosed no trace of drugs in the victim's system, Lt. Corral added. "I agree it's strange one, certainly stranger than many other cases we've had here."

One reason the findings took so long, according to the officer, was the effort investigators made to delve into Ms. Lam's medical history, which revealed her bipolar malady.

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He said the information was included in the finding because of Ms. Lam's odd behaviour in the elevator.

"This was a tragedy. She was a very young lady. It's unfortunate," Lt. Corral said.