Firefighters responded to the store at 1338 San Pablo Ave. shortly after 1 p.m., according to Pryor.
An employee picked up a backpack containing a bottle with some sort of liquid in it, and when the employee opened the bottle, several people immediately complained of respiratory irritation, Pryor said.
The incident caused five people to be transported to local hospitals, with four going to the Alta Bates Summit Medical Center in Berkeley, and the fifth going to the Kaiser Permanente Oakland Medical Center, she said.
Alta Bates spokeswoman Carolyn Kemp said two of the four victims taken there have already been treated and released, while the other two remained at the hospital as of 4:20 p.m. to be assessed by the medical staff.
The store reopened by 4 p.m., according to Pryor.
Investigators do not know what the substance was that caused the incident, but think it may have been some sort of pepper spray, Pryor said.
The person responsible for the backpack has not been located, she said.