Authorities say the device went off by the front door of the store at 1975 E. 17th Street just before 9 a.m. Less than a dozen workers were inside at the time.
One customer had just walked in through the front door when the explosion happened.
"He hears a clank clank, like metal landing on the cement, then he hears hissing, and then a loud boom and smoke, so he immediately ran inside, called us, and we immediately called the bomb squad," said Santa Ana Police Cpl. Anthony Bertagna.
A motive was unknown, but Bertagna said officers will be looking into whether there are any disgruntled employees due to the store closing.
The store opened to the public just after 11 a.m. The investigation is ongoing.