Deputy Commissioner John Miller said Thursday that Sayfullo Saipov is the only suspect, but that could change. The 29-year-old was charged Wednesday with terrorism and other crimes.
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Miller says he Saipov was "following ISIS instructions" to "yell it out" and spread the terrorist agenda.
Miller says the group suggests leaving leaflets at the scene of terrorist acts.
Eight people were killed in Tuesday's attack near the World Trade Center.
The victims included five people from Argentina, one from Belgium and two Americans, authorities said. Twelve people were injured and six remain hospitalized.
SCENE PHOTOS: Truck mows down crowd on bike path
Saipov's lawyer, David Patton, says he hopes "everyone lets the judicial process play out."
On Thursday, New York City Mayor Bill de Blasio and Schools Chancellor Carmen Farina paid a visit to Stuyvesant High School near the scene of the attack, where students were locked in place for hours
"We've talked about the strength and resilience of New Yorkers. There's an example of that today," the mayor said. "One of the students told me that in the hours that they were in lockdown they were checking on each other."
Farina previously talked to a student from another school who was hurt on the bus struck by the attacker's truck. He insisted on keeping up his perfect attendance record.
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Also Thursday, the medical examiner released his findings concluding that the eight victims died from blunt impact injuries to the head, torso and extremities. He ruled their deaths as homicides.
A married father of three and former commercial truck and ride-hailing driver, Saipov began planning an attack a year ago and settled on a truck assault a couple of months ago, according to a criminal complaint bringing terrorism charges against the Uzbek immigrant.
Police are looking at Saipov's background in the U.S. since arriving through a special visa program and when he was radicalized.
"It appears from what we know now, and this could change, is that he was radicalized after he came to the United States," said Miller.
Because he was captured alive investigators are anxious to talk to him at length, hoping to glean as much information as possible.
On Wednesday President Trump appeared to be referring to 23 people who listed Saipov as their point of contact when they entered the United States since 2010.
US officials tell ABC News that at least two of the 23 connected to Saipov were suspected of possible terror ties. This is of concern because a number of young men from Uzbekistan have been connected to ISIS in the past.
Miller, on "CBS This Morning", said "he seems to be referring to other people who came in that visa program. Part of that program is once you get here you have the opportunity to bring other family members."
Miller stressed that the 23 would not be considered part of the plot.
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It is possible that the 32-year-old Clifton New Jersey man who federal agents briefly asked the public to help them locate Wednesday afternoon was one of the 23, and that federal agents are working their way through those names to locate and interview those people.
That man, Mukhammadzoir Kadirov, is believed to be a friend of the suspect and is currently being interviewed by federal agents to learn what he knows about anything that led up to the terror plot.
The suspect's wife, Nozima Odilova, is also being interviewed by federal agents and is cooperating.
But the suspect is still believed to have acted alone.
Eyewitness News obtained video taken by a Stuyvesant High School student that shows the driver running erratically around in the street, in between passing cars, dressed in dark clothing and carrying the weapons.
This photo shows him in police custody following the incident:
(The Associated Press contributed to this report.)