Democratic Sen. Barbara Boxer is asking the Centers for Disease Control and Prevention to initiate a formal investigation into what has caused polio-like paralysis in about 20 children in California over the past 18 months.
Boxer says in a letter sent Thursday to CDC Director Thomas Frieden that "we need answers."
In particular, Boxer wants the agency to look into whether a virus is the culprit or if environmental factors are involved. She also wants to know if the agency has any way of knowing if there are other unusual reports of paralysis nationally.
Doctors at Stanford University said earlier this week that they had found some instances of children suffering paralysis in one or more limbs. They also sought to temper concern, saying the cases were a rare phenomenon.