The Iraq Survey Group was told that Saddam Hussein periodically removed guards from the Syrian border and replaced them with his intelligence agents who then supervised the movement of banned materials between Syria and Iraq, according to two unnamed defense sources that spoke with The Washington Times. They reported heavy traffic in large trucks on the border before the United States invasion. Earlier, in a telephone interview with "The Daily Telegraph", the former head of the Iraqi Survey Group, David Kay, said: "know from some of the interrogations of former Iraqi officials that a lot of material went to Syria before the war, including some components of Saddam's WMD program. Precisely what went to Syria, and what has happened to it, is a major issue that needs to be resolved." Satellite imagery also picked up activity on the Iraq-Syria border before and during the invasion. James R. Clapper, who headed the National Imagery and Mapping Agency in 2003, has said U.S. intelligence tracked a large number of vehicles, mostly civilian trucks, moving from Iraq into Syria. Clapper suggested the trucks may have contained materiel related to Iraq's WMD programs.