The company tells Action News that workers started coming back on Sunday night. Officials say they are providing an on-site counselor to help workers through the tragedy.
"We realize there is a tremendous amount of healing ahead of us and we're inspired by the strength and resilience of our colleagues," Kraft spokeswoman Susan Davison said in a statement.
The shooting happened Thursday night.
Yvonne Hiller, 43, had gotten into an argument Thursday evening with her co-workers at the northeast Philadelphia plant, police said. After she was disciplined, she went to her vehicle, made several phone calls, and then grabbed a .357 Magnum and used it to force her way past security guards at the front gate.
About 100 people were at work at the plant, which makes Nabisco cookies and crackers.
Hiller walked to the third-floor mixing area and found four people in the break room. She had no quarrels with one woman and told her to leave. She then opened fire on the others, police said.
Police identified the victims as Tanya Renee Wilson, 47; LaTonya Sharon Brown, 36; and Bryant A. Dalton, 39, all of Philadelphia. Wilson and Brown died at the plant.
After leaving the break room, Hiller went down a hallway and fired shots at the supervisor who had suspended her and at a mechanic who was using a walkie-talkie to alert police and co-workers to her whereabouts, police said. She missed both.
Hiller finally went to a second-floor office and called police to tell them what she had done, authorities said. Seven other workers were hiding in a closet nearby.
Hiller was charged with two counts of murder, one count of attempted murder, aggravated assault and other charges. She had a permit to carry the gun, authorities said.
Hiller has worked for Kraft for about 15 years, and had been in repeated arguments there in the past several years, a few of them physical, police said.