A package exploded Friday at a home on Bay Farm Island in Alameda, police and a United States Postal Service inspector said Sunday.
The package was sent through the Postal Service.
Inspector Jeff Fitch said among the responding agencies was a bomb squad and the Bureau of Alcohol, Tobacco, Firearms and Explosives.
Fitch said what happened is a federal offense.
Police and Postal Service officials were not yet releasing any more details.