SAN FRANCISCO (KCBS) -- San Francisco's fire chief is calling on residents of the city's Castro District to be on "heightened alert" after a string of fires hit the area in the early morning hours this week.

Investigators say five fires broke out under similar circumstances, usually outside apartment buildings, and may have been intentionally set. No one has been hurt, but thirteen people were displaced.

Fire Chief Joanne Hayes-White said her department and the police department are aggressively investigating the fires and are working hard to find whoever is responsible.

(© 2011 CBS Broadcasting Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed.)