The number of students enrolled in programs designed to aid autistic children in Minnesota has grown by 400 in the past year.

While that number has been steadily growing since the state Department of Education began collecting data on students with autism nearly two decades ago, the increase has tapered off in recent years, said Jonah Weinberg, executive director of the Autism Society of Minnesota.

The department's annual Child Count report shows that about 17,000 children up to age 21 were identified as having an autism diagnosis in 2014. In 2013, the number was about 16,600.

The report includes only students who are enrolled in an Individualized Education Plan as a result of their diagnosis, Weinberg said, so those who have been diagnosed but don't have a specialized plan at school aren't included.

Weinberg said a number of factors could contribute to the increase. For example, he said, as more instructors and caregivers have learned to detect subtle signs of autism, more kids are eligible for the education plans.

"Kids weren't getting screened as rigorously in previous decades," Weinberg said, "So that child who may have seemed a little withdrawn or shy or introverted may have flown under the radar before."

At the same time, if a student benefits from a specialized education plan and stops needing that support, he or she won't be included in the data. Students older than 21 are likewise phased out of the data.

Minnesota's population increased by about 35,000 from 2013 to 2014, according to data from the U.S. Census Bureau, an influx that could have brought more students affected by the disorder.

"Nobody knows what has caused the increase over the past several decades," Weinberg said.

And there's no official count of people diagnosed with autism in the state because there's no mandatory registry, according to a news release from the state autism society.

But if the Centers for Disease Control and Prevention's estimate that 1 percent of the population has autism is any indication, about 53,000 Minnesotans would have the disorder, the release said.

Anne Millerbernd is a University of Minnesota student on assignment for the Star Tribune.