Los Angeles police Det. Wallace Tennelle gave his thoughts in a deposition.
The veteran detective was answering questions posed by a lawyer for the notorious Los Angeles hotel where the 21-year-old passed her final hours.
Tennelle took the call in February 2013 about a Canadian national was missing from the Cecil. And he had peered into the open hatch of the rooftop water tank and seen Lam's lifeless body.
"My opinion is that she fell off her medication, and in her state, she happened to find her way onto the roof, got into the tank of water," Tennelle told the lawyer.
"At the time, I think that water tank was maybe full. But as people used the tank, used water, unknown to her, the level was dropping to a point where she could no longer reach out and escape, and she died that way."
A tragedy linked to L.A. legend
Tennelle's deposition was filed last month in response to a lawsuit launched by Lam's parents, David and Yinna Lam, who claim the owners of the 91-year-old Cecil failed to properly control guest access to the roof and water tanks.
The new court documents provide insight into a mystery that has only grown in the nearly two years since Lam stepped anxiously off a hotel elevator and into popular imagination.
The B.C. student's fate has inspired movie screenplays, an episode of Castle and — most recently — the fifth season of American Horror Story.
Tourists flock to the hotel. A four-minute surveillance video of Lam gesticulating to an off-screen presence and frantically pressing buttons before sidling off the elevator is the subject of endless internet speculation.
And by sheer coincidence, elements of the tragedy are linked to L.A. lore.
The detective who reviewed the tape is a consultant on the TV version of Michael Connelly's gritty Harry Bosch thrillers. The serial killer known as the Night Stalker, Richard Ramirez, reportedly lodged at the Cecil. So did Jack Unterweger, an Austrian serial killer and minor celebrity.
"Elisa Lam was young, pretty, on her own, in a country not her own," says L.A. crime historian Joan Renner.
"I don't think people are deliberately callous, but when they sink their teeth into something they feel is mysterious or otherworldly, they lose the importance of the actual victim in it. It becomes a puzzle."
The facts are as follows: Lam made internet reservations to check into a shared room for three nights on Jan. 28, 2013.
She was first assigned 506B, but the hotel's general manager said Lam's roommates complained about her "odd behaviour." She was then moved to a private room.
According to the coroner's report, Lam had a history of bipolar disorder. Medication was found among her belongings, but tests were inconclusive as to the presence of the medication in her bloodstream.
Her parents reported her missing when she failed to contact them on Jan. 31.
Tennelle set up a command post at the hotel when police intensified the hunt days later. He testified that "every nook and cranny of that building where we thought was a room, locked or unlocked, it was to be opened. It was to be searched."
The detective said Lam's appearance in the elevator video wasn't the only time she was seen on hotel surveillance tape.
"We did see her come in with two gentlemen. She had — they had a box, gave it to her," he said. "She went up into her — to the elevator. We never saw them again on video."
The search included the roof, but came up empty. Lam's belongings had been moved to the basement: a backpack, laptop and "things of value" that led police to believe she had planned on returning.
And then, nothing.
Maintenance man Santiago Lopez discovered the body on Feb. 19. A guest in room 320 complained about the lack of water pressure and he went to check out the four large rooftop tanks.
There are only four ways onto the roof: three fire escapes on the sides of the hotel and one alarmed door connected to an interior staircase.
The hotel's engineer said he tested the alarm regularly. It was in working order when Lam went missing.
"I think she went through the door," Tennelle told the Cecil's lawyer.
Lam was naked when her body was found, but Tennelle said her clothes were in the tank. They're detailed in the coroner's report: a green Alexander Keith's India Pale Ale shirt, black shorts and a red American Apparel sweatshirt.
The coroner concluded there was no evidence of foul play, and that "a full review of circumstances" didn't support the idea Lam intended to harm herself.
With 35 years on the force, Tennelle has investigated rape, robbery and murder; he's no armchair internet sleuth.
He also knows first-hand the effect of a senseless death on a family. His son Bryant was slain in an apparently random shooting in 2007.
Tennelle concluded his deposition in a lawyer's office 15 minutes away from the roof where Lam died.
"It was my opinion that she climbed in on her own," he said.
"My partner and I tried to figure out how somebody could have put her in there, and it's difficult for someone to have been able to do that and not leave prints, not leave DNA or anything like that. So she climbed in on her own."