WASHINGTON (AP) — Tests of pilots killed in plane crashes over more than two decades show an increasing use of both legal and illegal drugs, including some that could impair flying, according to a study released Tuesday by the National Transportation Safety Board.

The study examined toxicology reports for almost 6,700 pilots killed in crashes from 1990 to 2012. Not only did the share of pilots testing positive for a drug increase over that period, but the share of pilots who tested positive for multiple drugs increased as well. Pilots testing positive for at least one drug increased from 9.6 percent to 39 percent, while positive tests for two drugs rose from 2 percent to 20 percent and three drugs from zero to 8.3 percent.