British police have searched six addresses and made seven arrests in raids as part of an investigation into Wednesday’s attack on the Houses of Parliament which left four people dead and 40 injured, London’s Metropolitan Police has announced.

News of the police action, including one address in Birmingham, follows on from Wednesday when a suspected terrorist plowed a car into pedestrians on London’s Westminster Bridge and then fatally stabbed a police officer before being shot.

Mark Rowley, assistant commissioner of the Metropolitan Police, said at a press conference Wednesday night that the dead attacker was believed to be the only suspect, but declined to comment on the assailant’s identity. The police’s working assumption is that the attacker was inspired by international Islamist terrorism.

Londoners can expect to see heavier police presence, both armed and unarmed, throughout the city for the next few days, Rowley said.

Speaking Thursday morning, U.K. Defense Secretary Michael Fallon said that Parliament is currently reviewing Parliament’s already stringent security measures.

Wednesday’s attack unfolded around 2.40 p.m. local time when the assailant drove a grey Hyundai i40 along a pavement on Westminster Bridge, before running to Parliament. There he was confronted by a police, including unarmed PC Keith Palmer, who later died from stab wounds.

Tobias Ellwood, a member of Parliament who was at the scene at the time, was praised for his efforts to provide mouth-to-mouth resuscitation to Palmer. The other victims are yet to be named.

Addressing the country outside 10 Downing Street Wednesday evening, Prime Minister Theresa May condemned the attack and applauded the heroism demonstrated by the police.

“The terrorist chose to strike at the heart of our capital city, where people of all nationalities, religions and cultures come together to celebrate the values of liberty, democracy and freedom of speech,” she said.

“But let me make it clear today, as I have had cause to do before, any attempt to defeat those values through violence and terror is doomed to failure.”

Londoners and tourists will wake up on Thursday and go about their lives as normal, May said.

The city’s mayor, Sadiq Khan, echoed the prime ministers remarks, saying “Londoners will never be cowed by terrorism.”

The attack is the worst London has seen since 2005, when the capital was hit by a series of suicide bomb attacks on public transport during rush hour on July 7.

The city has been on “severe” alert since 2014, which indicates that an attack is “highly likely”, in response to conflicts in Iraq and Syria. This was an increase from a previous level of “substantial”, which implies a strong possibility of an attack.

Parliament is to hold a minute’s silence at 10 a.m. London time on Thursday to commemorate the victims of the attack.

Wednesday marked the one year anniversary of suicide attacks which took place in Brussels’ Zaventem Airport. Memorial services were held around the country to remember the 32 people killed and 320 injured by the attack.