A quick google of "material witness" yields, sadly, an appeal concerning the material witness warrant of Terry Lynn Nichols which was issued in connection with the grand jury proceeding against Timothy McViegh.

"The warrant stated on its face that "the Witness's testimony is material in a criminal proceeding, and he has attempted to leave the jurisdiction of the United States, and it may become impracticable to secure his presence by subpoena."

Apparently, a material witness is one whose testimony is material to the case, but might flee if subpoenaed and must therefore be arrested.

http://www.law.emory.edu/10circuit/feb96/95-3130.wpd.html

Sparteye?