Chipotle is planning to reopen 43 restaurants in Washington and Oregon weeks after an E.coli outbreak left dozens in two states sickened, despite not knowing the source of the outbreak, which health officials said was likely linked to the chain.

In total 27 people in Washington and 15 people in Oregon have been diagnosed with E.coli infection and health officials said "most" reported eating at a Chipotle restaurants before getting sick.

This week, the Chipotle restaurant chain announced it will reopen 43 locations that were voluntarily closed last month, after undergoing a strenuous cleaning process, testing and replacing ingredients.

Health officials in Washington said tests are still in progress but that they may not discover a source for the outbreak.

"Food outbreak investigations do not always identify a specific food source" the Washington Department of Health said in a statement. "A common reason for this is that the contaminated food source was consumed before the food samples were collected."

Symptoms of E.coli infection often occur around three to four days after exposure, although it can take up to 10 days for symptoms to appear, according to the CDC. Chipotle said it believes a meal served on Oct. 24 was linked to the E.coli outbreak.

Chipotle said in a statement the restaurants would brig in new ingredients come in and that testing will be done on produce, meat and dairy before restaurants are restocked. Additionally all restaurants nationwide will undergo deep cleaning and sanitation. They said no employee had been sickened.

“The safety of our customers and integrity of our food supply has always been our highest priority,” Steve Ells, chairman and co-CEO of Chipotle, said in a statement. “If there are any opportunities for us to do better in any facet of our sourcing or food handling – from the farms to our restaurants – we will find them. We are sorry to those affected by this situation, and it is our greatest priority to ensure that we go above and beyond to make certain that we find any opportunity to do better in any area of food safety.”

Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center, said that Chipotle's limited menu may have made it more difficult for investigators to detect the source of the outbreak. If a number of menu items share ingredients, it can be difficult to pinpoint which is the culprit for the outbreak.

"What you really need in order for the finger to be pointed at the source is for people to eat different things: People who eat A get sick and people eat B do not," Schaffner explained to ABC News. "Many of the ingredients, lettuce for example, can be on any number of different products."

Additionally, he said that foodborne outbreaks can occur with low level contamination that can be difficult to detect. For example, a vegetable may test negative still have bacteria that could potentially sicken someone.

"A lot of foodborne outbreaks are due to low-level contamination of product,” Schaffner said, explaining that testing is not always foolproof. "The testing...and cultures come back negative and [there] could still be low-level contamination."