LONDON — Police on Friday announced two “significant’’ arrests in connection with the attack outside the British Parliament this week, as they sought to unravel the murky past of the 52-year-old assailant, who they said was born Adrian Russell Ajao.

Mark Rowley, assistant commissioner of the Metropolitan Police, said that nine people were in custody and being questioned as part of the investigation into the assailant, who was identified on Thursday by another name he used, Khalid Masood.

Rowley said the toll of fatalities from the attack had risen to four with the death overnight of Leslie Rhodes, 75, from the Streatham area of South London. The victims included at least 50 wounded and came from around the world, a “pointed reminder,’’ Rowley said, of the global reach of the assault.

As a portrait began to emerge of Masood as a man who fluctuated between violence and a seemingly prosaic family life, Rowley said police were focusing on several questions regarding the assailant, who had never been convicted of terrorism: What led him to be radicalized? Had he acted alone? Did he receive direction from a source at home or overseas?

On Thursday, the Islamic State claimed responsibility for the attack, but the extent of the militant group’s connections to the assailant were unclear. 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.

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.

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 passersby on Westminster Bridge at the time of the attack, 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.