The Office of the State Fire Marshal is investigating a fatal fire that started early Sunday at 43 St. Lawrence St. in the city’s Munjoy Hill neighborhood. All four units of the three-story apartment building were occupied when firefighters responded just after 2 a.m. and crews encountered heavy fire conditions, the fire marshal’s office said in a news release Sunday night. The Portland Fire Department told people to avoid St. Lawrence Street in a Facebook post early Sunday morning as crews fought the fire. Six investigators were on scene throughout the day working to determine the cause of the fire, said Maine Department of Public Safety spokesperson Shannon Moss. Anyone with information or video of the fire is asked to contact the Portland Police Department at 207-874-8575 or the Maine State Fire Marshal’s Office at 207-626-3870.