A nationwide outbreak of a respiratory virus last fall sent droves of children to emergency departments. The infections have now subsided, as researchers knew they would, but they have left behind a frightening mystery.

Since August, 103 children in 34 states have had an unexplained, poliolike paralysis of an arm or leg. Each week, roughly three new cases of so-called acute flaccid myelitis are still reported to the Centers for Disease Control and Prevention.

Is the virus, called enterovirus 68, really the culprit? Experts aren’t certain: Unexplained cases of paralysis in children happen every year, but they are usually scattered and unrelated. After unusual clusters of A.F.M. appeared this fall, enterovirus 68 became the leading suspect, and now teams of researchers are racing to figure out how it could have led to such damage.

“It’s unsatisfying to have an illness and not know what caused it,” said Dr. Samuel Dominguez, an epidemiologist and an infectious disease specialist at Children’s Hospital Colorado, which has had the largest cluster of patients.