EMPLOYEE STATED THE IGNITION OCCURRED IN THE NO.2 ENTRY. IT WAS THE 1ST ROOF BOLT INSTALLED IN THE NO. 2 ON THE EVENING SHIFT. DRILL HOLE WAS DRILLED NEXT TO A FRACTURE & BOLT WOULD NOT TIGHTEN. BASED ON THE INFO. BY THOSE PRESENT, IT APPEARED TO IGNITE WHEN A SPARK FROM THE TIGHTENING IGNITED AN UNKNOWN METHANE BLEEDER IN THE ROOF FRACTURE.