LONDON - Police in Britain have arrested seven people in overnight raids in London, Birmingham and elsewhere in connection with the Wednesday attack on the Houses of Parliament by a knife-wielding man who first drove a vehicle down a crowded sidewalk.

Speaking Thursday morning, the top anti terrorism officer for the London Metropolitan Police (Scotland Yard), Mark Rowley, revised the death toll in the attack down by one, saying two civilians -- a woman in her 40's and a man in his 50's -- were killed, along with a police officer and the attacker. On Wednesday, Rowley had said three civilians died of their injuries.

“Greater clarity is now developing regarding the casualty figures as we have now collated information from the public and five hospitals,” Rowley said Thursday.

CBS News has learned that an American man is in critical condition as a result of the attack. His wife was treated for less serious injuries. The couple have not been identified.

He also said 29 people were wounded, including seven who remained in critical condition on Thursday. That toll was also significantly lower than reported the previous day in the wake of the attack, when Rowley said 40 people were injured.

British media quoted witnesses as saying the suspect had lived at a home raided in Birmingham, which is about 120 miles north of the capital city.

“The man from London lived here. They came and arrested three men,” a man who works near one of the raided buildings told the Press Association news agency. Officials did not confirm the specifics of the raid at that location, or any of the six others.

The suspect, yet to be publicly identified, was fatally shot by an armed police officer after he stabbed the first officer he encountered just inside a gated entrance to the Parliament campus. Police Constable Keith Palmer died of his stab wounds.

Prime Minister Theresa May said Thursday in the House of Commons that attacker was British-born, and known to intelligence agencies. 

The civilian deaths occurred on Westminster Bridge, which spans the River Thames. The attacker drove an SUV down the wide sidewalk of the bridge, plowing through throngs of tourists and other people going about their business in the heart of central London on Wednesday afternoon.

May said the victims include 12 Britons, three French students, two Romanians, four South Koreans, a German, one Pole, one Chinese, one American and two Greeks. It wasn’t immediately clear why the prime minister listed only one American as injured.

The Associated Press contributed to this report.