The blaze broke out Friday night in Palo Alto at a home on Embarcadero near Greer Road.
Firefighters found two bodies inside.
ATF investigators along with the Palo Alto and Sunnyvale fire departments were among those combing the scene Sunday.
No one would say what they were looking for or how the fire started.
Crews were hoping to enter the home Saturday but said the structure was too unstable.
A next door neighbor says a man and his caretaker lived here.
Their names have not been released.