LONDON — Authorities on Thursday identified a 52-year-old Briton as the man who mowed down pedestrians and stabbed a policeman to death outside Parliament, saying he had a long criminal record and once was investigated for extremism — but was not currently on a terrorism watch list. The Islamic State claimed responsibility for Wednesday’s attack, which police said was carried out by Khalid Masood, a British-born resident of the West Midlands in central England. Masood plowed a rented SUV into pedestrians on London’s Westminster Bridge, killing an American man and a British woman and injuring more than 30 people of almost a dozen nationalities. Police arrested eight people — three women and five men — on suspicion of preparing terrorist acts as authorities sought Masood’s motive and possible support network.