Four people have died, including a policeman and the attacker, in London outside of Parliament in the terrorist attacks that left 20 people injured.

From Fox News:

Police said a vehicle mowed down pedestrians on London’s Westminster Bridge, leaving more than a dozen with injuries described as catastrophic.

Around the same time Wednesday, a knife-wielding attacker stabbed a police officer and was shot on the grounds outside Britain’s Parliament, sending the compound into lockdown

The British police began “a full counter-terrorism investigation” after the attack. London Mayor Sadiq Khan confirmed that officials will treat the situation “as a terrorist attack until the police know otherwise.”

Acting Dep. Commissioner and Head of Counter Terrorism Mark Rowley announced that officials believe the attacker worked alone. Three officers also were injured.

The attack caused officials to lock down Parliament and shuttle away Prime Minister Theresa May to safety. Acting Metropolitan Police Commissioner Craig Mackey described what happened:

He said that the incident started at 2.40pm and that the investigation was responding to reports of “a person in the River Thames, a car in collision with pedestrians on Westminster Bridge and an incident involving a man with a knife”. He said it was “ongoing” and appealed for members of the public with footage of the incidents to come forward.

He added that he could not confirm the number or nature of injuries sustained.

President Donald Trump reached out to May and “pledged the full cooperation and support of the United States Government in responding to the attack and bringing those responsible to justice.” The State Department announced that its officials will monitor the situation and warned Americans to stay away from 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.

“Our hearts go out to those affected,” Toner said.

French officials confirmed that three French schoolchildren were injured during the attack:

The three students from the Concarneau Lycee in Brittany, western France, were on a school trip there, a foreign ministry spokesman said in a statement. The statement did not say how serious the injuries were.

A total of 90 students from the school were in London, about a dozen of which were in the area of the attack, an official at the school told Reuters.

“As the centre of London was hit by a cowardly attack, France assures the British people of its solidarity,” the foreign ministry statement said, adding that French authorities were in contact with their British counterparts.

Witnesses described the attack to the media:

Sky News reported a woman was pulled alive from River Thames.

The Port of London Authority says a female member of the public was recovered from the river, injured but alive.

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.”