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 U.S. Centers for Disease Control and Prevention (CDC), said on the call.