A series of incidents near British Parliament in London has claimed the life of four people and injured several others including a police officer who was stabbed by an attacker in what police are calling a terrorist attack. Among the dead is the assailant, police say. U.K. Prime Minister Theresa May condemned it as a “sick and depraved terrorist attack.”

Around the same time the police officer was stabbed, a vehicle struck as many as 20 people on the nearby Westminster Bridge, killing two, according to the BBC. The suspect was shot dead by police.

Police believe there was only one lone attacker, the Associated Press reports. But London Metropolitan Police counterterrorism chief Mark Rowley said police believed there was only one attacker, “but it would be foolish to be overconfident early on.”

The threat level for international terrorism in the U.K. was already listed at severe, meaning an attack is “highly likely.” No known terror group has claimed responsibility for the attack, officials say.

Other reports of the chaos include a woman being pulled out of the nearby Thames River to be treated for her injuries, hundreds of people being moved from Parliament as a safety measure and at least 10 people being treated on Westminster Bridge.

Daily Mail journalist Quentin Letts told the BBC he saw a man in black attack a police officer outside Parliament before being shot two or three times as he tried to storm into the House of Commons.

“He had something in his hand, it looked like a stick of some sort, and he was challenged by a couple of policemen in yellow jackets and one of the yellow-jacketed policemen fell down and we could see the man in black moving his arm in a way that suggested he was stabbing or striking the yellow-jacketed policeman,” he said.

The incident in London unfolded within sight of some of the city’s most famous tourist sites, including the London Eye, a large Ferris wheel with pods that have views over the capital. It stopped rotating and footage showed the pods full as viewers watched police and medical crews on the bridge, which has Big Ben at its north end.

Britain’s Prime Minister Theresa May will chair a meeting of the government emergency committee to discuss the response to the terror incident in London.

The emergency committee known as COBRA coordinates the high-level response to serious incidents. It brings together government ministers with senior officials of the emergency services and security and intelligence agencies.