Officers searched four Alpha Project sites Wednesday morning after shelter officials received an emailed bomb threat.
The email, to Alpha Project headquarters, stated that there was a bomb inside a porta-potty, but was not specific regarding which location, according to OnScene.TV.
All shelters were evacuated following the threat, at about 9:10 a.m., and San Diego police officers conduced searches at each site. They found no explosives.
Officials re-opened the shelters about an hour later.
Alpha Project has offices or shelters in several communities, including the East Village, Hillcrest and the Midway district.