Janesville police are investigating an early morning burglary at an east side retail store.  Janesville police responded to the Dollar General Store in the 2200 block of West Court Street a little before 2:30 a.m. Monday.  Officers say a rock was used to smash out a window.  While a police K-9 was tracking the suspect, several pieces of evidence were recovered.  Police also say they have suspect photos thanks to video surveillance.