There seems to be a lot of confusion regarding the history, purpose, and actual law of the material witness statute, 18 U.S.C. 3144. In 2005, the Congressional Research Service published a characteristically helpful summary of the material witness statute: "Arrest and Detention of Material Witnesses:Federal Law In Brief and Section 12 of the USA PATRIOT and Terrorism Prevention Reauthorization Act (H.R. 3199)." I found a copy and have uploaded it here.