LONDON - Four people died in a car rampage and knife attack in London, including a police officer guarding Parliament and an attacker, police said. 

A vehicle mowed down pedestrians on London’s Westminster Bridge Wednesday around the same time a knife-wielding attacker stabbed a police officer and was shot on the grounds outside Britain’s Parliament, sending the compound into lockdown. 

Mark Rowley, the head of counterterrorism at London’s Metropolitan Police, said some 20 people were wounded. A search was underway to make certain no other attackers are in the area, although police believe there was only one attacker. 

“We are satisfied at this stage that it looks like there was only one attacker,” Rowley said. “But it would be foolish to be overconfident early on.”

Rowley said the dead policeman was one of the armed officers who guard Parliament and that the other victims were on Westminster Bridge. 

Authorities said they were treating the attacks, which took place in the same area, as a terrorist incident.   

There was no immediate claim of responsibility. London Police Commander B.J. Harrington said a full counter-terrorism investigation was underway.

The area in which the incidents took place is home to popular tourist sites, CBS News Jonathan Vigliotti reports. The area was blocked off and metro stations in the area were shutdown as an investigation was underway.

The London Eye, a large Ferris wheel with pods that have views over the capital, stopped rotating and footage showed the pods full as viewers watched police and medical crews on the bridge, which has at its north end Big Ben and Parliament, two iconic symbols. 

“The whole length of the bridge there were people on the ground,” Richard Tice, a witness, told Sky News. The London Ambulance Service said it had treated at least 10 people on the bridge.

“There were people across the bridge,” said Colleen Anderson of St. Thomas’ Hospital. “There were some with minor injuries, some catastrophic. Some had injuries they could walk away from or who have life-changing injuries.” 

British port officials say they pulled a woman from the Thames river. She was injured but alive after the attack.

Romain Nadal, a spokesman for the French Foreign Ministry, said three students from Saint-Joseph high school in Concarneau who were on a school trip were among the injured.

London attack witnesses describe assault on Parliament police officer
London’s Metropolitan Police said they were called at approximately 2:40 p.m. local time to reports of the incident on Westminster Bridge, which is located just outside the Palaces of Westminster, home of the Parliament. 

Rick Longley told the Press Association that he heard a bang and saw a car plow into pedestrians and come to a crashing stop. Images from the scene showed pedestrians sprawled on the ground, with blood streaming from a woman surrounded by a scattering of postcards. 

“They were just laying there and then the whole crowd just surged around the corner by the gates just opposite Big Ben,” he said. “A guy came past my right shoulder with a big knife and just started plunging it into the policeman. I have never seen anything like that. I just can’t believe what I just saw.” 

At Parliament, a body was seen lying in the yard. It wasn’t clear if it was the attacker. 

Prime Minister Theresa May, who had been in the Parliament complex when the incident occurred, was whisked away from the area and was safe. 

U.S. Secretary of State Rex Tillerson expressed his condolences to the victims and their families on behalf of the United States.

“The American people send their thoughts and prayers to the people of the United Kingdom,” he said in a statement. “We condemn these horrific acts of violence, and whether they were carried out by troubled individuals or by terrorists, the victims know no difference.”

The White House said earlier that U.S. President Donald Trump was briefed on a gun and knife incident at Britain’s Parliament in London. Trump himself said during a brief appearance Wednesday before reporters at the White House that he was just getting the news. He called it “big news.” 

Trump’s spokesman, Sean Spicer, said that the U.S. will continue to monitor the situation and update the president. 

The U.S. State Department said it is closely monitoring the incident outside London’s Parliament and urged Americans in London to avoid the area. 

Spokesman Mark Toner said Wednesday: “We stand ready to assist in any way the U.K. authorities would find helpful.” 

He added that the U.S. Embassy in London is closely following the news and stands ready to help any affected Americans. 

He said: “Our hearts go out to those affected.”   

The incident comes on the one year anniversary of a double suicide bombing attack on the Belgian capital, Brussels, that left more than 30 people dead. ISIS was behind that attack.

The Associated Press contributed to this report