Forensics experts Thursday searched on their hands and knees for microscopic clues to the rampage that so far has claimed four lives, including the attacker. A British-born man first struck down pedestrians on Westminster Bridge with an SUV and then fatally stabbed a police officer guarding Parliament's courtyard in Wednesday's attack. Magnus Ranstorp, an expert on extremist tactics with the Swedish Defense University, said Parliament's buildings are well-protected but that little can be done to stop an individual from using a vehicle to attack pedestrians and then enter the grounds on a suicide mission. The grounds are protected by a security perimeter, with a mixture of armed and unarmed police, but the vehicles pass much closer to the buildings than is the case, for example, with the White House or U.S.