In 1992 Mattes served as Investigative Counsel to the US Senate POW Committee. As Investigative Counsel, Mattes conducted hearings exposing fraud and phony POW charity fundraisers After leaving service to the Senate, Mattes represented the Lost Army Commandos, a group of CIA assets left in POW camps for decades. Between 1959 and 1964, 450 South Vietnamese commandos–recruited, trained and paid by the US government for a secret mission known as Operation 34-Alpha–were captured in North Vietnam and Laos. Some were executed, others were imprisoned and tortured. The last commando held captive was released in 1988.
Mattes began searching for them in 1993 and two years later filed suit against the government for their back pay. The Defense Department initially refused to acknowledge the commando mission, but Mattes discovered 500,000 pages of classified documents at the National Archives. The documents included pay rosters indicating that the Army listed the commandos as killed in action – when they actually were in prison. The Army gave widows and survivors a minimal death payment of a few hundred dollars to close the cases. By declaring the prisoners dead, the Army no longer had to pay the families $2,000 a year while the commandos were in prison. It also kept the mission secret. The commandos acted as spies and saboteurs who infiltrated Communist territory.
After a 3 year court battle Mattes won a 20 million dollar settlement for the Lost Army Commandos. In addition, Mattes also assisted in winning asylum for POW families and successfully sued the CIA and Department of Defense on their behalf. The 23 million dollar settlement provided for over 350 POWS.