WASHINGTON — Democratic Sen. Barbara Boxer asked the federal Centers for Disease Control and Prevention on Thursday to initiate a formal investigation into what has caused polio-like paralysis in about 20 children in California over the past 18 months.

Boxer said “we need answers” in her letter to CDC Director Thomas Frieden. In particular, she wants the agency to look into whether the illness can be traced to a virus or environmental factors. She also wants to know whether the agency is aware of similar reports of paralysis nationwide.

“These questions must be answered because it is deeply disturbing to read reports of otherwise healthy children experiencing sudden paralysis,” Boxer said in her letter.

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.

“At this time, CDC does not think the situation in California is a cause for public concern,” Jamila Jones, a public health educator for the agency, said in an emailed statement. Her comments were not in direct response to the letter.

Jones said the CDC is concerned about the children who suffered what is described as acute flaccid paralysis, but the number of cases being investigated by California health officials fell within the expected rate within the overall population.

So far, state officials have not identified any common causes to suggest the cases are linked.