Since the cause of autism is unknown, the authorities in Minnesota say it is hard to know even what to investigate.

Image TO SCHOOL Ahmed Essa, 8, is autistic. A former Minneapolis school official said Somalis made up 25 percent of children in preschool classes for those with severe autism. Credit... Allen Brisson-Smith for The New York Times

“There are obviously some real concerns here, but we don’t want to make a cursory judgment,” said Buddy Ferguson, a health department spokesman. Even counting autism cases is difficult because the diagnoses are first made by the schools, not doctors, and population estimates for Somalis vary widely. Results are expected late this month.

Even if the department confirms that a cluster exists, it will not answer the question why. Still, Dr. Thun said a possible focus in one ethnic group “increases my sense that investigating it is essential.” The next step, he added, would be to look at Somalis in other cities.

A small recent study of refugees in schools in Stockholm found that Somalis were in classes for autistic children at three times the normal rate.

Calls to representatives of Somali groups in Seattle and San Diego found that they were aware of the fear in Minneapolis but unsure about their own rates. Doctors familiar with the Somali communities in Boston and Lewiston, Me., had heard of no surges there.

“It’s a concern here, but we haven’t done anything to look specifically,” said Ahmed Salim of Somali Family Services in San Diego.

Shamso Yusuf of the Refugee Women’s Alliance in Seattle said tearfully that her own daughter had been given a diagnosis of autism, “and I see a lot of parents who have 5-year-olds who cannot speak.” But no Seattle study has been done, she said.