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(CNN) —  

The federal investigation into what exactly is causing a peculiar outbreak of vaping-related lung injuries nationwide could continue for months.

The ongoing investigation “may take a few months” and could yield “multiple causes and potentially more than one root cause,” Dr. Anne Schuchat, principal deputy director at the US Centers for Disease Control and Prevention, said during a briefing with reporters on Friday.

“So I do think that the phenomenon we’re seeing is going to have an explanation, but it may not be tomorrow,” she said. “It may take a few months to really understand the portion of illness that’s due to some new risky practice in the preparation of these materials or other causes.”

CDC, US Food and Drug Administration, state and local health departments, and other clinical and public health partners are investigating the multistate outbreak of lung injury associated with use of e-cigarette, or vaping, products.

“We are going to leave no stone unturned to try to get to the bottom of this,” Mitch Zeller, director of the FDA’s Center for Tobacco Products, said during Friday’s briefing.

“This is an extraordinarily complicated investigation with a great diversity of products and intervening acts or actors that could be modifying these products along the way, especially for the great majority of the cases that involve THC and the presence of oils and other compounds,” he said.

All patients in the outbreak have reported a history of using e-cigarette products, according to the CDC, and most have reported a history of using products containing tetrahydrocannabinol or THC, the primary psychoactive component of cannabis.

Last week, the FDA warned consumers to stop using THC vaping products as investigations into vaping-related lung injuries and deaths continue.

The agency is not pursuing “any enforcement actions associated with personal use of any vaping products,” rather the agency’s “interest is in the suppliers,” FDA Acting Commissioner Dr. Ned Sharpless said in a statement released last week.

“If we determine that someone is manufacturing or distributing illicit, adulterated vaping products that caused illness and death for personal profit, we would consider that to be a criminal act,” he said in the statement. “We are prepared to use our authorities to the fullest extent possible, and will work with other federal, state and local authorities to take appropriate action as the facts emerge in order to protect the public health.”

The CDC on Friday released updated guidance for health workers caring for people with vaping-related lung injuries. In the guidance, the CDC refers to the e-cigarette, or vaping, product use associated lung injury by a new acronym, EVALI.

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Schuchat said on Friday that a “handful” of lung injury patients have been readmitted to hospitals from five to 55 days after they were discharged.

Patients might have returned to vaping, or might have been vulnerable to other illnesses after their lung injury, she said, noting that “the issue of readmissions is a relatively new consideration in the outbreak.”

As of Tuesday, there have been 1,299 lung injury cases associated with e-cigarette products in 49 states, the District of Columbia and the US Virgin Islands, according to CDC. Alaska is the only state without a reported case.

Individual states have reported a total of 29 deaths: three each from California and Indiana; two each from Georgia, Kansas and Oregon; and one each from Alabama, Connecticut, Delaware, Florida, Illinois, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Nebraska, New Jersey, New York, Pennsylvania, Texas, Utah and Virginia.

More deaths are under investigation.