Transcript for 8 dead in NYC after man mows down pedestrians, cyclists with truck
since 9/11. Reporter: On Halloween, terror in Manhattan. Multiple casualties. There is a mass casualty situation here. Reporter: This video showing the man accused of going on a rampage on a crowded bike path in New York City. Taking at least eight lives. Five visitors from Argentina and injuring 18 more, leaving nearly a mile of wreckage in his wake. Reportedly done in the nail of ISIS. Now being investigated as a deliberate act of terrorism. Only blocks away from one world trade center. We have been tested before as a city very near the site of today’s tragedy. And new yorkers do not give in in the face of these kinds of actions. Reporter: The ordeal began just before night fall as families were out trick or treating. I see a truck a few more blocks down. Reporter: The suspect, sayfullo saipov, driving a pick-up truck rented from home deep over and taking what witnesses described as a deliberate turn down a bike path. I saw a couple of bodies in the bike path. Reporter: Mowing down bicyclists and pedestrians. Some bystanders at first miss taking the bloodshed as a Halloween performance. It was Halloween. Reporter: The suspect only stopped after crashing into a school bus. All I heard was the impact of a crash and then I saw the white pickup truck. Reporter: The video shows him going out into the street, dodging vehicles waving what authorities said were mock guns. He was in the street. A whole bunch of people were keg, he’s got a gun. He was getting out of the car. He did look like he was dragging his foot a little bit. He was screaming in the seat and he looked frustrated and confused. Reporter: Sources say he was shouting, Allah akhbar. It was one after another after another. Reporter: Among frightened witnesses, children. It really freaked me out. I was crying and really scared. I was like this. On the ground. Reporter: The NYPD quickly arriving on the scene, confronting him and then firing at him. Striking him in the abdomen. He was then apprehended and is currently being treated at an area hospital. He wanted to initiate a suicide by cop. Those guns were not lethal womens. I believe he he anticipated being shot and killed. Reporter: Ambulances rushed in, triaging the wounded among the carnage. Be advised. There are multiple people on the ground. From chambers all the way to Houston. All the way from Houston. Reporter: Police heavily armed and with canine teams quickly established a perimeter around the area. The crime scene extending for more than ten blocks. And you can see sole officers are still there. This road ISD closed off. It leads right to the scene of the attack. And investigators will be coming through here all night long. Near nearby the high school. I saw the man with the gun. He pointed the gun at me. I’m sorry. I have to go get my child. Reporter: To Tony clayfield, the scene was reminiscent of another dark day. The first thing that came to my mind was September 11. I knew it was terrorism. My first thought was my daughter. Going to get her for safety. Reporter: Yvette attends community college in the area, dressed for Halloween herself. She told us she saw many kids on the street in costume. When I saw the little kids coming out of school, my heart broke down. I felt like, what’s going on? What if it would have happen right here where the school is with all these kids. What would have happened next? Reporter: Tonight authorities have discovered a letter allegedly written by saipov saying he acted on behalf of ISIS. They believe he acted alone but there are many questions. What caused him to basically commit this horrible act today? And there will be some version of a tick tock as to why he got to where he ultimately ended today. Killing a number of people in New York City. And they’re going to want to figure that out. The reason why is, you’re always looking for the pieces to stop the next guy. Reporter: What we do know now is he is believed to be originally from Uzbekistan. Emigrating to the U.S. Through the Visa program. Now word that saipov was an Uber driver. Uber saying, we are horrified by this senseless act of violence. Investigators believe he recently lived across the river from Manhattan at this house in Paterson, New Jersey. That he previously lived in Ohio and his license was issued in Florida. Local police swept an apartment they think belongs to the suspect, searching for answers. The attack mirror what’s leaders have ISIS have publicly called for. Are you incapable of stabbing someone with a knife? Throwing someone off a building or running they will over with a car? It is very difficult to stage large scale terrorist attacks with a cell of five or six people. When there are that many people involved, they can be identified in advance. Someone can make a mistake. A lone actor, particularly one who hand done anything before. One that the police are not following. A lone actor can make this kind of big publicity event. Reporter: Today’s incident, the latest in a string of vehicle attacks. This August, Harris used a van to kill zpleen injured more than 100 more in Barcelona. That driver escaped but was later in a shoot-out with police. From March to June there were three separate attacks in London, claiming a total of 15 lives. Last December in Berlin, a driver killed 12 people in a Christmas market. And in nice, a man used a 20-ton truck to kill 84 revelers celebrate celebrating bastille day. It is very hard to stop someone from using a truck and attacking a crowd at any intersection in a major city. Reporter: In may, the TSA released a report recommending reporting to authorities any suspicions arising from the rental of large today that is at this vehicles near large gatherings, and creating barriers that create a serp Entin path of entry. We visited over 148 truck rental locations in this area. The obvious ones, youhaul, Ryder, et cetera. The industry has had a high level of awareness on this matter from the NYPD. Reporter: President trump briefed on the events today tweeting, it looks like another attack by a very sick and deranged person. Law enforcement is following this closely. Not in the usa. New York governor Andrew Cuomo and New York mayor bill de Blasio standing together to reassure the city. We will be vigilant. More police everywhere. You will see them in the airports and the tunnels. It is not because there is any evidence of any ongoing threat or any additional threat. It is just out of vigilance and out of caution. But tonight, New York with its history deeply marred by terrorism remains a city on edge. Reporter: Halloween appears to be carrying on as usual in times square. We are seeing a lot of costumes, a lot of tourists and a lot of police. Reporter: Not far from the crime scene, new yorkers in costume coming out in full force carry out the Halloween parade tradition. It can happen anywhere. So could it make us a little afraid. But I mean, we have to live, right? You can’t let something tragic that someone did stop your life. Reporter: The indomitable spirit of the city, brave and unbroken. For “Nightline” in New York.
This transcript has been automatically generated and may not be 100% accurate.
