A large majority of the more than 1,600 people who have been sickened said they vaped products containing THC, the high-inducing ingredient in marijuana, but it's likely something else — or perhaps several things — are to blame, officials say.
"We don't have a short list" of suspect substances, Mitch Zeller of the U.S. Food and Drug Administration told reporters Friday. And there may be more than one cause of the illnesses, he said.
The outbreak appears to have started in March. Illnesses have been reported in every state except Alaska, many of them teen and young adults. At least 34 have died.
The Centers for Disease Control and Prevention has been defining cases through a diagnosis of exclusion: People who vaped and developed severe breathing problems are counted when viruses and other potential causes are ruled out.
At times, it's been unclear if doctors were seeing a single condition. Some reports suggested patients had "lipoid pneumonia," with their lungs clogged and inflamed by fatty oils. Others have pointed to the kind of chemical burns that might come from poisonous gases.
But microscopic images may be pointing to another understanding of how vaping can disrupt the lungs' delicate balance.
A study published this week focused on certain white blood cells that were collected in the damaged lungs of eight THC-vaping patients.
Researchers concluded the cells did not appear to have sucked up fatty oils inhaled from outside the body, but rather the debris from cells that died after some chemical exposure. The patients, they believe, were suffering different manifestations of the same thing.
"It's a question of how severe the injury is, not what the injury is," said Dr. Sanjay Mukhopadhyay, a Cleveland Clinic pathologist and the paper's lead author.
What chemical might have caused it? He can't say.
Of potential causes, so far "nothing lines up 100 percent," said Dr. Sean Callahan, a University of Utah Health lung specialist who has treated vaping illness patients.
Callahan was among doctors who have described the illness as lipoid pneumonia. But with new information coming in, he said he is ready to stop using the term because it implies the patient inhaled fats.
Dr. Peter Briss, who is managing the CDC's investigation, said experts are still sorting out the differences in what doctors are describing.
"The understanding is still evolving, and we're doing additional work to try to further characterize this," he said.
The number of new cases each week seems to be declining, but health officials say it's not clear whether the outbreak is leveling off.
They voiced concern that a new burst of cases could appear as flu season sets in, because people who damaged their lungs by vaping may be unusually susceptible to respiratory viruses.
Electronic cigarette users could be "more likely to get sick and stay sick," said Dr. Michael Matthay, an expert in acute respiratory distress at University of California San Francisco.