Transcript for Deadly attack causes chaos in the heart of London

as we come on the air, the death toll rising from that deadly attack in London. A single attacker using a car as his weapon, mowing down people on a bridge to the British parliament. People running for their lives. You see it here. That iconic building put on lockdown. One of the victims end up in the river below. The attacker crashing his car, then going after a police officer. Stabbing him. Authorities finally stopping the attack with a bullet. The alleged terrorist did not survive. But four of his victims are also dead, including that police officer, and at least 40 people are injured. Tonight, the desperate search for clues and a motive. ABC's Alex Marquardt reporting from London takes us inside the attack. Reporter: Chaos in the heart of London today. Panicked crowds in the packed streets outside the palace of Westminster. Tonight, the attack being treated as a "Terrorist incident" that instantly paralyzed Britain's parliament. People fleeing as gunshots ring out. Police shouting at pedestrians to run. Go! Go! Reporter: Authorities quickly arrived at the scene, locking down parliament. This house is now suspended but please wait here. Reporter: The attack unfolding just before 3:00 P.M. On the busy Westminster bridge, just before rush hour. The suspect's car careening onto a sidewalk. Plowing through pedestrians, killing at least two and injuring around 20 others. There was bodies literally -- There must have been, like, ten bodies. 10, 12 bodies. In different places along the bridge. Reporter: Moments later, that car crashing into this gate outside parliament, just below big Ben. The car smoking, its hood smashed, the driver's side door open as crowds begin to form. I see that one car smashed in the wall and just people started running all over. Reporter: From above on the famous London eye, these tourists watching the carnage below. We were in the eye. We saw different groups of wounded people on the bridge. One of the things that struck us was how long medics were spending with the victims on the ground. Reporter: Authorities say the suspect then exits the car to continue the deadly rampage. Inside parliament's grounds, stabbing and killing a police officer who was standing guard. One witness watching from a window. We saw a thick-set man in black clothes. Had something in his hand, it looked like a stick and we could see the man moving his arm in a way that suggested he was either stabbing or striking the policemen. Reporter: Police fire, taking the suspect down. These images and immediate aftermath show one officer seeming to point his gun at the alleged suspect. What appears to be a knife under the officer's right foot. Police load the alleged suspect onto a stretcher and into an ambulance. The suspect later dying. Parliamentarians on armed guard. Theresa may rushed to safety. Outside, that bridge brought to a standstill. Bystanders rushing to a victim's aid. First responders performing triage as helicopters were dispatched to rescue the wounded. Tonight, a search is under way to make sure there were no other attackers as stories of heroism begin to eearnimerge. To bias Ellwood with his face bloodied, performing mouth to mouth on the officer that later died. Such a grizzly scene. Alex is joining us outside of London now, and Theresa may as addressed her nation saying they believe the suspect acted alone? Reporter: They believe this attacker acted alone, and they have not identified the suspect and there has been no claim of responsibility. Prime minister may called this a sick and depraved terrorist attack, adding that londoners will never give into terror, and parliament will be back in session here tomorrow. Tom? It will be a long night for you, thanks so much.

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