A baroness was in a committee room, listening to a presentation about African development. A lord was in the Gilded Chamber, waiting for a 3 p.m. session on topics including school gardens and the digital economy. A member of Parliament was walking through an underground passageway, on his way to a vote.

Then, on the south bank of the Thames, a traffic light turned from red to yellow to green, and a gray Hyundai began racing across Westminster Bridge toward the center of the British government. This was the beginning of an afternoon of carnage that would leave four unsuspecting victims dead and at least 28 others injured in the deadliest terrorist strike in this capital since 2005.

The assailant was eventually shot and killed by police. On Thursday, a stunned Britain learned that he was a 52-year-old born in this country — a man who had been investigated for extremist ties in the past, and was believed to be motivated by radical Islamist ideology. He was identified as Khalid Masood.

This account of his trail of destruction is based on the recollections of eyewitnesses as well as of individuals who experienced the attack’s aftermath in Parliament and on the streets outside.

(Jason Aldag,Victoria Walker/The Washington Post)

It was a brisk Wednesday afternoon in London, almost spring-like, when the vehicle began roaring across Westminster Bridge about 2:30, according to witnesses interviewed by British and European media. One person later recalled it zig-zagging. Pedestrians scattered in panic as the careening vehicle jumped the curb, plowing into them.

One woman was pinned under a bus that ground to a halt. Another woman toppled off the bridge into the river below. “We believe she fell,” a spokesman for the Port of London Authority said in a statement, adding that she was quickly rescued.

“I was just walking across the bridge and suddenly a bus stopped, and everybody started screaming,” Steve Voake, a children’s book author, told the BBC. He noticed a sneaker by the side of the bridge, “and then on the other side of the road there was a body, and when I looked further up, there was another body, and then when I looked over the side of the bridge, there appeared to be a body in the water as well.”

Emergency services kicked into gear at 2:40 p.m.

At least three members of the public who were on the bridge were killed— a 54-year-old musician from Utah, Kurt Cochran, who was celebrating his 25th wedding anniversary; Aysha Frade, 43, a British teacher and mother of two; and a 75-year-old man who died of his injuries Thursday and was not immediately identified. Three police officers were struck on their way back from a commendation ceremony. Among the injured were several French teenagers on a school trip to London, as well as numerous other tourists.

The vehicle exited the bridge, traveled a short distance and crashed into a railing encircling Parliament.

(The Washington Post)

That’s when Andrew Woodcock, political editor for the Press Association with an office on the third floor of the Parliament building, heard a loud bang. Screams and shouts followed. He rushed to his window, which overlooks New Palace Yard, a courtyard situated to the northwest of the Houses of Parliament and adjacent to Big Ben clock tower and Parliament Square.

“It was horror,” Woodcock later recalled.

About 40 or 50 people were running away from Westminster Bridge, around the corner into Parliament Square. As they dashed past the courtyard’s entrance, a man appeared to emerge from the throng, wielding a knife, “maybe nine inches long,” Woodcock said.

As the man waved the knife above his head, the police officers guarding the entrance to New Palace Yard backed away from him, through the gates into the courtyard.

As Woodcock ducked into his office, he heard people shouting “armed police” in an apparent warning to the assailant. The journalist then heard three or four gunshots.

Meanwhile, a Taiwanese tourist was outside, recording video of the pedestrians running away from New Palace Yard. “They shot him,” someone yelled, according to the video, later posted to Twitter.

Woodcock returned to the window. He saw two people lying on the ground — apparently the fallen officer and the attacker.

“Within seconds there were armed police all over New Palace Yard,” he said.

Photos coursed through social media of Tobias Ellwood, a member of Parliament, trying to resuscitate the policeman, Keith Palmer, who had been fatally stabbed.

Ambulances arrived in Parliament Square, and paramedics took over first-aid efforts.

“I saw Tobias Ellwood being led away from the scene straight beneath my window, blood on his hands, blood on his face, looking very shaken,” Woodcock said.

As the attack unfolded, Parliament was locked down. Traffic was halted. The London Eye — a large Ferris wheel alongside the Thames — came to a halt, passengers stuck in the pods. Officers and paramedics carried victims to the nearby St. Thomas’ Hospital.

Grant Shapps, a Conservative member of Parliament, was in a walkway by the House of Commons when he heard what sounded like four gunshots ring out. The next thing he knew, he was surrounded by armed officers who shouted, “Get back, get back, get back!” He retreated to the chamber on hands and knees.

“I came up and told the deputy speaker, ‘You have to suspend this vote because MPs won’t even be able to get here, let alone the fact that something very serious has happened,’ ” he recalled. He then waited for several hours in the members’ lobby within Parliament.

Over fruit and water, lawmakers found solace in talking to one another, Schapps said.

“You’re told as an MP that this could happen — and the advice is not to be a hero,” said Calum Kerr, a member of the Scottish National Party. He was walking through an underground passageway to the Parliament when he saw people running frantically back into the walkway.

He made his way to the building, and from there he was taken by officers first to a nearby courtyard and eventually on to Westminster Abbey, where hundreds of people — peers, members of the House, staffers — took sanctuary and gave testimony to the police.

“In an awful lot of countries, they’d have had a riot in there, and we just sort of carried on,” Lord Digby Jones said of the atmosphere in the church. He had been shepherded out of the Lords Chamber, where he was waiting for a session to begin, shortly after 3 p.m., told only that an “incident” was unfolding.

Pola Uddin, another member of the House of Lords, was leaving the central courtyard for Westminster Hall when she saw a covered body on the ground. She only later found out that an officer had died in the assault.

London had faced terrorism threats for years, she said — and had been lucky.

“We have lived under these circumstances for quite some time now,” Uddin said. “And you have to think: Many of these [attacks] have been prevented.”

Rick Noack contributed to this report.