This has just been reported a few hours ago, no details have been released besides what is in this from a link to the report from Reuters:

Five members of a family, including three children, were found shot dead at their southern Maine home on Sunday, with a gun located near one of the bodies and no outside involvement suspected, a police spokesman said.

The five, a father and mother in their 30s and three children believed to be 4, 7 and 12, were discovered on Sunday afternoon in three rooms of their second-floor apartment in Saco, about 17 miles (27 km) southwest of Portland, state police spokesman Stephen McCausland said.