US health officials said on Friday there may be more than one cause behind the nationwide outbreak of serious lung illnesses linked to vaping, and that they have not yet seen a meaningful drop in new cases. Investigators have pointed to vaping oils containing THC, the psychoactive ingredient in marijuana, as being especially risky, but have not yet tied the cases to any specific product or compound. In a conference call with reporters, health officials said more than one agent may be involved in the mysterious illness that has sickened more than 1,000 people in the United States. “I think there will be multiple causes and potentially more than one root cause,” Dr. Anne Schuchat, principal deputy director of the US Centers for Disease Control and Prevention (CDC), said on the call. The source of the illness in one part of the country may not be the same as in another part of the country, she said. On Tuesday, the CDC reported 1,299 confirmed or probable cases of lung injuries linked to vaping in the United States, with 26 deaths, including one victim under the age of 18. About 80% of patients were under age 35. The CDC issued updated guidance for doctors, urging them to be especially watchful of patients who may have both a vaping lung injury and an unrelated respiratory infection. The CDC had previously advised doctors to rule out infections before considering vaping injuries, but that was during the summer months when there are fewer cases of respiratory infections such as flu. “It may be more complicated to understand now that we expect to see a higher degree of respiratory infections,” Schuchat said. The CDC is now advising doctors to devise treatments that would address both conditions. Schuchat said a handful of vaping patients who recovered and were discharged were later readmitted to the hospital, but it was not clear whether these individuals resumed vaping and incurred a new injury. The new guidelines urge doctors to follow up with patients a week after discharge and to begin vaping cessation treatments or therapies for THC and nicotine before patients leave the hospital. Dr. Ned Sharpless, acting commissioner of the US Food and Drug Administration, said the agency has begun testing on more than 700 product samples and product parts gathered from patients and health officials. FDA scientists are testing for a broad range of chemicals, including nicotine, THC and other marijuana components, metals, cutting agents and other additives, pesticides and toxins. “Based on our testing of samples to date, there does not appear to be one product or substance involved in all of the cases. It may be there is more than one cause to this outbreak,” he said on the call. “We do know that THC is present in most of the samples tested to date,” Sharpless said. Mitch Zeller, director of FDA’s Center for Tobacco Products, said the agency has evaluated 225 THC-containing products and found Vitamin E acetate – a cutting agent – in nearly half of them. Sharpless said the FDA is collaborating with customs and border control agents to intercept any products containing illicit substances. “If we determine that someone is manufacturing or distributing illicit, adulterated vaping products that cause illness or death for personal profit, we would consider that to be a criminal act,” he said. The CDC and the FDA continue to urge people to stop vaping, especially products containing THC.