Police said it appears the man, whose name was not released, was the victim of a homicide attempt and assault with a deadly weapon.
He was discovered in the area of Church and Duboce streets about 7:30 a.m. Sunday morning. He was reportedly critically injured and without a wallet or identification, health officials said.
Earlier today, police did not believe the subject was a victim of criminal activity, Officer Albie Esparza said, but later in the day evidence was introduced that authorities say showed attempted murder and assault with a deadly weapon.
The victim's neighbor recognized him after the hospital released his picture and called the man's family, who came to the hospital to identify him.
There was no suspect in custody Monday.
Anyone with information on a possible suspect is encouraged to call (415) 575-4444 or text TIP411 with “SFPD” at the start of the message.
Bay City News contributed to this report.