"On Friday, a parcel was delivered to a home in Alameda and it did detonate," said U.S. Postal Inspector Jeff Fitch.
Fitch said one person was injured in the blast but did not provide any details.
Law enforcement sources say a woman living in the home was hurt but will recover.
Several neighbors say the man who owns the home is a police officer.
"I think he works around Alameda. He's very nice," said neighbor Maricar Rodriguez.
Neighbors did not report hearing an explosion but saw numerous police cars and an ambulance outside the house on Friday afternoon.
Fitch would not comment if there are any suspects in custody or where the parcel was mailed from and why.
The ATF, Alameda County Sheriff's department and Alameda Police are assisting in the case.