A third employee was wounded in the shoulder and neck, The Philadelphia Inquirer reported.
Police Commissioner Charles Ramsey said the woman, who had worked at the plant for 15 years, was back in the building less than 10 minutes after she was escorted out at 8:35 p.m. EDT. Ramsey said the woman, carrying a .357 Magnum, used her car to ram through a fence.
"I don't know what her motive is and I don't want to guess," Ramsey said.
Police did not identify the shooter, but fellow workers gave her name as Yvonne Hiller.
A police SWAT team rescued seven people who were hiding in a quality control room. All were unharmed.
Hiller was taken into custody. KYW News Radio reported she surrendered after calling police on her cellphone.
One employee told the Inquirer he heard someone yelling "Get out" followed by a bang. He said most workers ran out of the plant.
The plant, formerly operated by Nabisco, produces Ritz Crackers and Lorna Doone shortbread.