GREEN BAY, WI (WTAQ) - A Family Dollar store in Green Bay was robbed at knifepoint as it was closing its doors Sunday night.  Police say it happened around 9 p.m. at 550 North Military Avenue.  A masked man entered the store with a knife and demanded money and took off with undermined amount of cash.  Police say the man was wearing a black hooded sweatshirt, black pants and a black back pack with a red mask with eye holes cut out.  Police say the man was dark skinned, possibly African American.