Charter schools are enrolling a disproportionately low number of students with disabilities compared to traditional public schools, though it’s unclear what’s contributing to the disparity.

The finding comes from a Government Accountability Office report released Wednesday looking at the experiences of students with special needs in charter schools. Currently charters serve some 2 million American children and their influence is growing.

For the study, GAO looked at data from the U.S. Department of Education for the 2009-2010 school year, the most recent available. Just 8 percent of charter school students had disabilities that year compared to 11 percent of kids enrolled in public schools.

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Government investigators were unable to determine why fewer students with disabilities were attending charters, but said anecdotal evidence suggests a number of factors. Parents of children with disabilities may not be choosing charters or the schools may be discouraging such students from enrolling. Local schools districts could also be playing a role in student placements.

Meanwhile, some charters may be ill-equipped to serve students with severe disabilities, GAO said.

In visits to 13 charters in three states, investigators said they found that schools were publicizing and offering special education services, but officials at half of the schools said “insufficient resources” were a challenge.

“The charter school movement across the country is breaking down old stereotypes about which students can and can’t learn — whether poor, minority or a student with a disability,” said Rep. George Miller, D-Calif., the top Democrat on the House Education and Workforce Committee. “As we move forward with education reform, we need to ensure that students with disabilities are a part of the educational revolution that is taking place within charter schools.”

Complaints from parents of students with disabilities about access to charter schools have been bubbling up for years. Such issues came to the forefront in New Orleans in 2010 with parents bringing a class-action lawsuit alleging that the city’s schools — most of which are charters — were denying access to students with disabilities.

The federal report, however, is among the first to offer a comprehensive look at how students with disabilities are faring at charters nationally.

GAO officials are recommending that the U.S. Department of Education issue further guidance to help charters understand their obligations to students with disabilities. What’s more, investigators say the agency needs to research what’s leading to enrollment disparities.