(CNN) -- The investigation of the E. coli outbreak linked to Nestle Toll House cookie dough is nearly over, according to the U.S. Food and Drug Administration. The cause of the contamination has not been identified.
Nestle recalled all its Toll House refrigerated cookie dough products made at a Danville, Virginia, plant.
David Acheson, the FDA's associate commissioner for foods, says the samples of cookie dough his agency has tested do not contain the E. coli strain that sickened 74 people in 32 states.
On June 19, Nestle recalled all its Toll House refrigerated cookie dough products made at a Danville, Virginia, plant. Thirty-four people have been hospitalized; no deaths have been reported. The CDC reports that the majority of the patients were teenage girls who reported eating the cookie dough raw.
Last month, the FDA found E. coli in a production sample of Toll House cookie dough, but after testing, investigators found that the strain in the sample did not match the outbreak strain.
"We are still in speculation mode," Acheson said, but the E. coli "most likely came from raw ingredients" such as flour.
The E.coli strain that caused the outbreak, 0157:H7, is typically found in fecal contamination from animals such as cows, sheep or goats. Acheson points out that it's not unusual for those animals to be carrying more than one strain of the bacteria. But he believes that the root cause of the contamination may never be known.
"We have to conclude we're unlikely to have a definitive determination," he said.
Most people with E. coli 0157:H7 experience diarrhea and abdominal cramps within eight days of infection.
The FDA notified Nestle USA on Thursday that its investigation at the Danville plant was over.
Nestle says FDA inspectors were at the plant for more than a week and found no E. coli on equipment. More than 1,000 tests have been conducted at the facility after the outbreak.
"We've dismantled the production line and done extensive testing on all equipment and not found any E. coli," Nestle USA spokeswoman Laurie MacDonald said.
MacDonald announced that Nestle is easing back into production at the plant. The company has purchased new lots of eggs, flour and margarine and says it is thoroughly testing the new products and all raw ingredients.
Consumers will have to wait to see cookie dough back in grocery stores.
"You're not going to see anything on the shelves for quite some time," MacDonald said. But when it does appear, the company will mark the cookie dough with a new label that clearly designates it as a new batch.
It will also continue to carry a reminder to not eat the dough raw.