A man was struck and killed by a Red Line train at Porter Square station

Authorities say the 66-year-old was walking inside the northbound tunnel.

Red Line train in Porter Square
Porter Square station. –David L. Ryan/Globe Staff

A 66-year-old man died Sunday after he was struck by a Red Line train at Porter Square station.

MBTA Transit Police said officers responded to a call for a person struck by a train around 7 a.m. The initial investigation indicates the man was “trespassing on the right of way approximately 10 yards into the northbound tunnel” when he was struck, police said.

Service on the line was temporarily shut down and replaced with shuttle bus service.

The male was pronounced deceased as a result of injuries sustained,” police said.

Authorities said “foul play” is not suspected at this time, but the incident and surrounding circumstances remain under investigation by police and the Middlesex District Attorney’s office.