A federal appeals court debated Monday whether government properly used DNA evidence, gathered years earlier, to win convictions against a man for threating the lives of a number of politicians, including former U.S. Senator Joe Lieberman.

The U.S. Postal Inspection Service collected Michael Thomas’ DNA while investigating the 2004 delivery of an envelope containing white powder to a parochial school in Massachusetts that he once attended. Thomas was cleared and the investigation was closed in 2006.