The Dole Food Company, one of the largest processors of fresh produce, knew it had a listeria problem in one of its salad plants more than a year before it closed in January, according to a Food and Drug Administration report.

The report, obtained by Food Safety News and The Food Poisoning Bulletin through the Freedom of Information Act, shows that products in the plant tested positive for listeria nine times before F.D.A. inspectors showed up to do a test in January at a plant in Springfield, Ohio.

Four people have died after becoming ill so far in the outbreak, and 33 more across the United States and Canada have become so sick they have spent time in a hospital. Dole said the Justice Department was investigating.

“They’d been having positive tests for listeria for some time,” said Bill Marler, a prominent food safety lawyer who represents one of the victims in a lawsuit against Dole. “If the government inspectors hadn’t showed up, who knows when or if they were going to tell anyone.”