The federal government should do more to prevent people from fraudulently obtaining health insurance subsidies under the Affordable Care Act, the U.S. Government Accountability Office said in testimony to the Senate Finance Committee.

The testimony, delivered on Thursday in person and in writing by GAO spokesman Seto Bagdoyan, caps an extensive investigation by the GAO which found that, as of last April, about 431,000 people were still allowed to receive subsidies for insurance purchased through the federal exchange in 2014 despite possible inconsistencies in their applications.