Ingrid Loyau-Kennett is a 48-year-old mother of two and a scout leader. When her bus passed by the chaotic scene, Kennet jumped off to help the victim and that's when she came face to face with one of his killers.
Loyau-Kennett told British media that she first thought the man lying in the street was in a car accident. She checked for a pulse and realized he was dead. One of the killers, who was holding a small axe and two knives, told her to move off the body.
Loyau-Kennett then realized the dead man had been hacked to death. She asked the man if he did it and why. He reportedly told her the man was killed because he had killed Muslims in Afghanistan.
The woman tried to keep talking to the two bloody attackers before police came, keeping them from lashing out at the crowds that were gathering.
Police eventually arrived at the scene and shot and wounded both suspects. The barbaric attack happened near the Royal Artillery Barracks.
A number of weapons - including butchers' knives - could be seen on the blood-spattered street.
The brutal attack appears to have been driven by radical Islam, according to two U.K. government officials.
The government officials, who were briefed on the situation, said the attack seemed to have been ideologically motivated by radical Islam. The officials spoke on condition of anonymity because they were not authorized to speak about the ongoing investigation.
British Prime Minister David Cameron said there are "strong indications" that the attack is related to terror.
French President Francois Hollande, speaking at a press conference in Paris with Cameron, said it was a British soldier who was killed. Cameron didn't immediately confirm that fact but the Britain's Ministry of Defense said it was urgently investigating if a U.K. soldier was involved.
One British broadcaster aired footage of what appeared to be one of the attackers, his hands covered in blood, making political statements about "an eye for an eye" to an unknown cameraperson as a body lay behind him on the ground.
The footage, which was obtained by ITV news, showed a man in a dark colored jacket and a knit cap walking toward a camera, clutching a meat cleaver and a knife in what appeared to be bloodied hands. He apologized in English for the women passers-by who "have had to witness this" attack, saying that "in our land our women have to see the same."
He gave no indication where that land was, though ABC News reports the man spoke with a southern London accent.
"We must fight them as they fight us," the man told the camera as people milled around behind him. The camera then panned away to show a body behind the man.
A few minutes later, armed police arrived. Witnesses told ABC News that the two men rushed the police, who opened fire, wounding both of them.
The attackers "went for the police with the machetes, knife and handgun," one eyewitness told the BBC. "I don't think they cared."
The prime minister's office said security was stepped up at barracks across London following the attack. Cameron cut his France visit short and called the attack "absolutely sickening" and a "most appalling crime."
"People across Britain, people in every community, I believe, will condemn this attack," he said at a news conference in Paris. "We have had these sorts of attacks before in our country and we never buckle in the face of them."
The barracks - which house a number of the King's Troop Royal Horse Artillery and independent companies of the Grenadier and Coldstream Guards - were the site of shooting events during the 2012 London Olympics.
The Associated Press and ABC News contributed to this report.