Key senators say the Federal Aviation Administration is stonewalling their attempt to get documents that might explain how the agency approved the Boeing 737 Max, which remains grounded more than a year after two deadly crashes.

Senate Commerce Committee Chairman Roger Wicker, a Republican from Mississippi, said Wednesday that the FAA has failed to respond to more than half of his committee’s requests for documents.

Wicker says it’s hard to believe that the FAA isn’t deliberately keeping Congress in the dark. FAA Administrator Stephen Dickson promises to work harder to cooperate with Congress. The 737 Max has been grounded since March 2017 after the second crash.