London police say at least five people have been killed in a terrorist attack at the British Parliament, including a suspected attacker, a police officer stabbed by the assailant, and "three members of the public" who were struck by the assailant's vehicle when he rammed into pedestrians on Westminster Bridge.
Police said they believed a lone assailant carried out the attack on March 22, which left at least 20 injured, but that they were continuing a counterterrorism investigation and had not ruled out the possibility that others may have been involved.
Britain's top counterterrorism officer, Mark Rowley, said that "Islamist-related terrorism" was suspected as the motive, but he provided no details.
The attack began at 2:40 p.m. local time, when a man drove a car at high speed across Westminster Bridge and rammed into a group of pedestrians -- killing at least two people -- before crashing the vehicle into a railing beside Parliament and London's iconic Big Ben clock tower.
Police said the attacker then got out of the vehicle and ran inside Parliament's security perimeter through a vehicle entrance gate and fatally stabbed a police officer.
He was then shot by plainclothes police officers and died after being taken away for treatment.
The police officer who was stabbed died later in a hospital.
Hospital officials said several people who were struck by the assailant's vehicle suffered "catastrophic" injuries.
One woman who fell from the bridge into the River Thames was pulled out alive by rescue workers, but was said to have suffered serious injuries.
French Prime Minister Bernard Cazeneuve confirmed that three French schoolchildren, aged 15 to 16, were among those injured on Westminster Bridge.
Three London police officers and two citizens of Romania also were among the injured who were struck by the vehicle on the bridge.
Westminster Bridge spans the River Thames and is used for vehicle and pedestrian traffic. It meets the Palace of Westminster compound near its northern end.
A spokesman for Prime Minister Theresa May said she was safe after the attack.
She had been in the House of Commons at the time of the attack and was quickly shuttled away by security to her office at 10 Downing Street, where she chaired an emergency security meeting at her Cabinet Office Briefing Room later on March 22.
May said after that meeting that the government will not raise its terrorism-threat level following the attack.
Instead, she said Britain's terrorism-threat level will remain at "severe" -- the second-highest level -- which means an attack is highly likely.
May also condemned the violence at Parliament as a "sick and depraved terrorist attack," saying its location was not an accident.
A full counterterrorism investigation was launched by London Metropolitan Police.
WATCH: Attack On British Parliament In London
Meanwhile, Parliament was placed on lockdown and lawmakers were sequestered in a secure area for about four hours along with others in the compound before people were allowed to leave.
In Washington, the White House said President Donald Trump offered May the full cooperation of the United States government "in responding to the attack and bringing those responsible to justice."
Earlier in the day, Secretary of State Rex Tillerson also expressed his condolences and said Washington condemned "these horrific acts of violence, and whether they were carried out by troubled individuals or by terrorists."
There was no immediate claim of responsibility for the attack.
WATCH: London Police Make First Statement On Attack
The violence outside the British Parliament came on the first anniversary of three coordinated suicide bombings in Brussels that killed 32 victims and injured more than 300 others.
Extremists in the Islamic State extremist group claimed responsibility for the Brussels attacks.
Authorities said later that the Brussels attacks were carried out by members of a terrorist cell that had been involved in the November 2015 Paris attacks that killed 130 people and injured 368 others.