Authorities from Great Britain to the U.S. are scrambling to unravel the mysteries behind deadly chaos that’s been declared a terrorist attack in London on Wednesday.
Five people are dead, including one police officer and the attacker who was gunned down outside of Parliament after ramming his car into pedestrians on Westminster Bridge.
Using a large knife, he had just killed a uniformed police officer who guarded the government compound.
Another 40 people are being treated for injuries -- some of the cases described as "catastrophic".
Britain's prime minister was hustled out of Parliament by a plainclothes security detail.
The rampage unfolded at 2:40 p.m. local time at the crossroads of national politics in Britain.
After accelerating into crowds of people on the bridge sidewalk, the driver kept going toward Westminster Yard and smashed into a fence.
Witnesses say the driver climbed out of the car, went inside Parliament's gates, and tried to enter the building.
A police officer was in his way, and wound up being repeatedly stabbed and killed.
A gunshot from a team of other officers reaching the scene brought the attacker down.
The curiosity of passersby quickly turned to shock, horror and sadness.
"I started to realize there were a lot more people roaming around and security,” said bicyclist Matt Haikin, “and (I was) just thinking maybe this is more than just a car accident."
It was, by then, a case of lockdown and national security.
Plainclothes officers rushed Prime Minister Theresa May out Parliament, and safety-bundled her into a car that took May to her office at 10 Downing Street.
Some members of Parliament hit the floor and crawled for cover, as tourists visiting the area were evacuated until police secured the scene.
A British foreign minister who was outside Parliament at the time tried to stop the stabbed officer's bleeding before paramedics arrived.
A woman in the path of the attacker's car avoided being hit by jumping over a bridge railing, into the River Thames.
So far, no radical group has claimed responsibility for the attack -- but some have encouraged supporters to use "any weapons at hand", including cars.