Police are probing the suspicious death of a 6-year-old girl and an injury to a boy who were both found in a Fitchburg home yesterday, Worcester District Attorney Joseph D. Early said.Early told reporters the girl was found “unresponsive” and a 9-year-old boy was found injured at a home on Stoneybrook Road.The girl was later pronounced dead at a hospital, authorities said.The children were found at the home about noon, authorities said.