Police hope the note's author will come forward with more information about the incident.
The name of the 18-year-old beating victim has not been released.
The investigation began Monday night with the message slipped under the door of the University Police Department on campus of the University of Alaska Anchorage. Chief Rick Shell said Thursday two officers were on duty at their desks in a squad room at about 8:30 p.m.
"One of them got up to do something and noticed a note on the floor," he said. "He picked it up, read it, let the other officer read it. The other officer saw content that led him to believe that the Anchorage Police Department should be involved."
They called Anchorage police and officers drove to the nearby abandoned home in downtown Anchorage, said police spokeswoman Anita Shell.
Officers found the young man in the basement. Police said he was unconscious in critical condition and being held against his will.
The home was owned by Covenant House, which operates a shelter across the street for homeless teenagers. Director Alison Kear said the home was scheduled to be knocked down Wednesday to expand a parking lot.
Wrought iron mesh had covered the windows and doors were locked, Kear said. A security company had patrolled outside. The only access she could guess was a second-floor window not much bigger than a dog door.
"How they were able to enter there is perplexing to me," she said.
Shell could not say Thursday whether the crime scene was the abandoned home or elsewhere.
"We have not yet spoken to the young man because of his medical state," she said.
In either case, she said, investigators allowed a scheduled demolition of the home to move forward and a wrecking crew used heavy machinery to knock the building down Thursday morning.
The man's injuries were extensive, Shell said.
"It's my understanding he was beaten around the head and the face, the whole body," she said.
University Police Chief Shell praised the quick response by both departments.
"This was just an absolutely outstanding case of two officers taking something that could have been fairly innocuous and taking it seriously," Shell said. "It ended up saving the young man's life."