This afternoon’s terrorist attack on the Houses of Parliament in central Westminster left four dead, including the attacker and a police officer, and twenty injured, some seriously. For the third time in a year, a lone killer has used a vehicle as a weapon on the streets of a major European city. Yet again, the target seems to have been selected for its symbolic resonance. Last July, it was the Bastille Day celebrations in Nice, France. In December, it was the Christmas market in Berlin. Today, it was the Mother of Parliaments.

Just before 2:45 p.m. GMT, while the House of Commons was debating a proposal for a referendum on Scottish independence, a silver Hyundai 4×4 veered off the roadway of Westminster Bridge, which meets the northern bank of the Thames under the iconic “Big Ben” clock tower. As usual, the narrow sidewalk was crowded with tourists and office workers. The car left bodies strewn in its wake. Witnesses reported that the impact threw two bodies into the Thames; one woman was retrieved badly injured but alive.