Authorities in London made two more "significant" arrests in the investigation into Wednesday's terror attack that sent shock waves through the country.

The latest sweep brought to nine the total of arrests made in the wake of the attack. One woman was released on bail, Reuters reported. Authorities are working to determine if the attacker, Khalid Masood, who died at the scene, was directed or acted alone.

Masood killed four in the attack outside British parliament. Police said they believe Masood, 52, acted alone when he drove an SUV into pedestrians on Westminster Bridge before stabbing a police officer on the Parliament’s grounds.

The dead included a British policeman, stabbed repeatedly, an American tourist who was celebrating his 25th wedding anniversary and a school administrator adored in the Spanish town where she spent summer vacations with her family. A 75-year-old victim of the bridge attack died late Thursday after he was taken off life support, police said.

Authorities said two officers are listed in critical condition. Reuters reported on Friday that the attacker’s birth name was Arian Russell.

According to police, Masood was born in Kent, U.K., and they believe he was most recently living in the West Midlands, which includes the city of Birmingham.

Masood was known to authorities and had a range of previous convictions for assaults, including grievous bodily harm, possession of offensive weapons and public order offenses.

ISIS claimed responsibility for the rampage, saying Masood was “an Islamic State soldier” who “carried out the operation in response to calls to target citizens of the coalition.”

London has been a target for terrorism many times over past decades. Just this weekend, hundreds of armed police took part in an exercise simulating a "marauding" terrorist attack on the River Thames.