MADISON, Wis. (AP) -

A fourth person is charged in an attack on Wisconsin star running back Montee Ball last summer.

An arrest warrant has been issued for 27-year-old Karlis Griffin. A criminal complaint says Griffin stomped on Ball's head during the attack as Ball was walking home with friends at bar time. Ball suffered a concussion and says he doesn't remember the attack.

The State Journal says unlike the three others charged in the assault, Griffin is not a University of Wisconsin-Madison student and has no permanent address. Griffin is charged with substantial battery.

Two of the three others charged have pleaded guilty to substantial battery and have been placed in a first-offenders program. The third is scheduled for a plea and sentencing hearing on Friday.