Windows in the building along an alley between Grand Avenue and Thomas Street near Noyes Street blew out about 6:50 p.m., police and fire authorities said.
Witnesses said the apartment was on fire, but flames were out by the time firefighters arrived, San Diego Fire-Rescue spokesman Maurice Luque said.
Luque said paramedics found one man injured in the explosion on the damaged second-floor of the apartment.
Another victim in the explosion had fled the scene, but later required medical attention from paramedics at Mission Boulevard and Toulon Court in Mission Beach.
Luque said one man suffered serious injuries while the other man’s injuries were considered moderate.
Authorities said both men were taken to a hospital for treatment.
The incident was being investigated by the Police Metro Arson Strike Team and hazardous materials crews.
— Dave Schwab