Public charter schools, a small but fast-growing segment of K-12 education, enroll fewer children with disabilities than traditional public schools, according to a new federal study.

The report, released Wednesday by the Government Accountability Office, examined how many disabled students are served by charter schools as compared with traditional public schools.

About 8 percent of the students at charter schools are disabled and require special services, compared with 11 percent of students in traditional public schools, the GAO found. Differences in enrollment were seen across a range of disabilities, from autism to speech impairment.

Public schools that accept federal money, including charters, are required by law to provide a “free appropriate” education to all disabled children. They cannot exclude disabled students or otherwise discriminate against them.

When it comes to children with intellectual disabilities, traditional public schools had almost twice as many as charter schools, according to the study.

“It’s troubling — we need to know why,” said Rep. George Miller (Calif.), the ranking Democrat on the House Education and the Workforce Committee, which requested the GAO study.

The findings were based on school data from 2008 to 2010. More than 2 million students attend about 5,600 charter schools across the country, according to the National Alliance for Public Charter Schools.

Todd Ziebarth, vice president for state advocacy and support at the alliance, questioned the data, saying statistics can be misleading. Some traditional public schools may be mislabeling struggling students as special-needs children, inflating their share, he said. Still, charter schools could improve, Ziebarth said.

“There’s been some progress made but still some challenges that remain,” he said.

The question of whether charter schools educate similar numbers of disabled students is significant as the charter movement grows, because critics have accused charters of “creaming,” or preferring to enroll students who are easier and less costly to educate.

The Education Department’s Office of Civil Rights is in the middle of a broad review of whether charter schools are complying with federal law regarding disabled students.

The GAO report said it is unclear why charter schools are enrolling fewer disabled students. Parents of disabled students may think that a charter school won’t adequately meet their children’s needs, it said. Or some charter schools may be discouraging students with disabilities from enrolling, the report said.

About half of the charter school officials the GAO interviewed cited insufficient resources, including limited space, as a challenge. Often, severely disabled children are removed from regular classrooms and given additional help in a separate room, but charters often lack the necessary physical space.

The GAO study recommended that the Education Department update its guidance to charter schools, addressing practices that may affect enrollment of students with disabilities. It also wants the department to determine the reason behind the enrollment disparity.