All the autistic Somali children in the study had I.Q. deficits, Dr. Hewitt said.

Even though the city has Asian and Native American communities, records for so few of those children were studied that they were not included in the analysis, she added, “but it’s reasonable to extrapolate that autism rates among them are lower.”

Autism rates vary widely across the 14 communities the C.D.C. follows, Dr. Boyle says. Alabama has low rates, while Utah’s and New Jersey’s are high.

Generally, says Michael Rosanoff, a director of public health research for Autism Speaks, white children are the most likely to have an autism diagnosis, but that may be because they are more often sent to diagnostic specialists.

Somali parents in Minneapolis have complained for years that many of their children had autism symptoms — failure to speak, reluctance to look others in the face, screaming and repetitive behaviors.

At onetime, 25 percent of the children in local special education classes were Somali, while Somalis represented only 6 percent of the student body. While some children back home had the same problems children everywhere do, parents said, autism was so unfamiliar that there was no Somali word for it until “otismo” was coined in Minnesota.

“I feel good, actually,” Idil Abdull, a Somali mother of an autistic child who was one of the first to demand an official investigation, said when she heard the results. “I was afraid they were going to say, ‘We don’t see anything.’ And we know that our kids can’t talk.

“Autism is silencing the kids of a nation of poets,” continued Ms. Abdull, who has spoken about the issue at the United Nations. “Whether it’s something in our genes and you add it to Minnesota snow or what, I don’t know, but something’s triggering autism. My dad taught me to recite poetry at age 4, and my kid is 11 and he can’t say two sentences. It’s heartbreaking.”

Dr. Hewitt and Mr. Rosanoff say they want to see more research comparing Somali children with autism to those without, including intelligence testing and genetic workups.