Updated

The Uzbekistan native accused of killing eight people in New York City had been planning his attack for weeks and did prior reconnaissance before he plowed a rented truck into pedestrians along a popular Manhattan bike path, officials revealed Wednesday.

Sayfullo Saipov, 29, was interviewed by investigators at the hospital after his surgery Wednesday, John Miller, NYPD’s Deputy Commissioner of Intelligence and Counterterrorism, said in a news conference. Miller did not reveal what was said, though reports indicated Saipov bragged about the assault and said he was “proud” of the attack. NYPD Commissioner James O’Neill would not confirm the statement.

Days before Tuesday's assault, ISIS had been encouraging its followers to carry out Halloween attacks with propaganda from the terror group showing a graphic with a blood-splattered machete and Tuesday's date. SITE intelligence group, which monitors terror activity online, also highlighted a recent photo taken by an ISIS supporter near the scene of Tuesday's attack; but it remained unclear if there was a connection between that photo and the rampage.

Sayfullo Saipov MUGSHOT

Sayfullo Saipov is accused of killing eight and injuring 12 in a terror attack in New York City. (St. Charles County Department of Corrections)

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Investigators have uncovered several pieces of evidence linking the “lone wolf” attacker to ISIS. Miller said Saipov, who came to the U.S. in March 2010 through the Diversity Visa Program, had been planning the assault for “a number of weeks,” toured the bike path area and did it “in the name of ISIS.” He also cited the notes left in the truck contained a pledge of allegiance to ISIS. The notes were handwritten and had symbols and Arabic words on them stating: “Islamic State would endure forever.”

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Sayfullo Saipov allegedly rammed his rental truck into bicyclists and pedestrians in Lower Manhattan Tuesday. (AP)

He added Saipov followed “almost to a T” the instructions that were put out by ISIS on how to carry out a vehicle attack. Several knives were also recovered from the rental truck, however, Saipov is not believed to have used them during the assault. FBI agents swarmed Saipov’s Paterson, N.J., home and were spotted leaving with several bags of evidence. Officials added that relatives were being interviewed, but would not further comment on it.

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Two sources told Fox News that Saipov’s name “popped” up in connection with a handful of individuals who were known to law enforcement when investigators ran his name through their databases after Tuesday's attack. It is not clear whether these investigations were limited to the New York City area or included Florida, where Saipov had lived. One source said Saipov had “close ties...[to] the group he rolled with [who] was known to law enforcement.”

New York Gov. Andrew Cuomo said that Saipov was radicalized in the U.S. and called him a “depraved coward” whose attempt to “instill terror” among New Yorkers failed.

“There was no grand statement that was done. It was an act of a coward,” Cuomo said.

O’Neill said Tuesday’s assault was the worst terror attack in New York City since Sept. 11, 2001. He added “no one in this city is complacent” and urged New Yorkers to stay vigilant in reporting suspicious activity. Cuomo, O'Neill and New York City Mayor Bill de Blasio assured that Sunday’s marathon will go on as planned with extra precaution.

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Police investigate a pickup truck used in an attack on the West Side Highway in lower Manhattan in New York City, U.S., November 1, 2017. (Reuters)

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There will be increased police presence at major infrastructures and on the streets as a precaution in the next few days. NYPD Chief Carlos Gomez added New Yorkers should expect more bag checks and additional officers in the subway and major hubs. There will also be more sand trucks — used to create a blockade to prevent vehicle attacks — placed along the marathon route on Sunday.

Saipov allegedly sped down the bike path next to Stuyvesant High School in a rented Home Depot truck late Tuesday afternoon before crashing the vehicle into a school bus. He jumped out, carrying a pellet gun and paintball gun in his hands, and screamed “Allahu akbar” — “God is Great” in Arabic — before NYPD officer Ryan Nash shot him in the stomach. Cuomo called Nash a “hero” for taking down the assailant.

Another 12 people were seriously injured. Three people were released from the hospital on Wednesday. Four remain critically injured and the others are seriously injured. The injuries ranged from bilateral amputation to serious head, body and leg trauma.

Among the eight people killed were five Argentinian men — identified as Aril Erlij, Hernan Diego Mendoza, Diego Enrique Angelini, Alejandro Damian Pagnucco and Hernan Ferruchi — and a woman, Anne Laure Decadt, who was from Belgium. Darren Drake, 32, from New Jersey, and Nicholas Cleves, 23, from New York, were identified as the two American citizens killed.

President Trump on Wednesday called for “Merit Based immigration” after officials revealed Saipov came in through the “Diversity Visa Lottery Program.”

Fox News' Catherine Herridge and The Associated Press contributed to this report.