At a National Transportation Safety Board hearing on the crash, officials said Theron Smith, the pilot, should not have had his license reinstated after a 2006 stroke, Politico reported.
Stevens, R-Alaska, Smith and three other people were killed in the crash last year. Four passengers survived.
The plane, an amphibious single-engine de Havilland Otter, went down near the small village of Alegnagik on Aug. 9. At the hearing, NTSB officials said investigators have been able to find no sign of mechanical trouble with the plane.
Deborah Hersman, the NTSB chairwoman, said Smith suffered a short period of "temporary unresponsiveness" but investigators were unable to find a cause. His wife said he was alert and well that morning and had fully recovered from the stroke.
"What we do not know -- and may never know -- is what happened in the last three minutes of that fatal flight," she said.
Hersman called for the installation of flight data recorders on small planes. The board also said the Federal Aviation Administration acted inappropriately when it restored Smith's license in 2008.