A 56-year-old woman was killed when her car went off a bridge and plunged into the Concord River in Lowell Monday morning, police said. Police and firefighters responded to the Lawrence Street bridge at 6:08 a.m. and discovered a motor vehicle had smashed through the railing of the bridge, Lowell police Captain Kelly Richardson said in a statement. Police believe the woman, who was identified as Colleen Luther of Lowell, “hit a Jersey barrier and careened across the street and through the railing.” Firefighters entered the water in an attempt to save Luther, but rescue was difficult because of the cold water and poor visibility, he said. They were eventually able to remove Luther from the vehicle, Richardson said, but she was pronounced dead at Lowell General Hospital. She was the sole occupant of the car, he said.