On Thursday, the Islamic State claimed responsibility for the attack, but the extent of the militant groupâs connections to the assailant were unclear. Mr. Masood, who was widely identified in British news reports late Thursday by another name, Adrian Elms, threw the heart of the capital into chaos on Wednesday when he drove a sport utility vehicle into pedestrians on Westminster Bridge and then killed a police officer with a knife outside the Parliament complex.
Mr. Rowley emphasized that there was no evidence of further security threats, adding that the counterterrorism operation was large and moving quickly, with hundreds of officers mobilized.
The police were searching five addresses, had concluded 16 searches and were sifting through 2,700 seized items, including huge amounts of computer data, and video footage taken by passers-by on Westminster Bridge at the time of the attack, Mr. Rowley said.
One person was arrested overnight in the West Midlands region of England, where several people had been detained earlier, and a second was taken into custody in the northwest of the country, he said. Another individual was released.
The authorities had held off releasing information about Mr. Masood, and Mr. Rowley thanked the news organizations that had delayed publishing…