Saipov, 29, was charged with providing material support to the Islamic State (ISIS) militant group and violence and destruction of motor vehicles.
He is so devoted to the ISIS that he wanted to display the terror group's flag in his hospital room, a 10-page complaint filed by the FBI before a federal court here against him shows.
Eight people were killed and 11 others injured as his truck carved a path of destruction through several blocks of Lower Manhattan yesterday. Saipov crashed the truck into a school bus, left the vehicle brandishing imitation firearms. He was shot by police. It was the the worst terrorist attack in on the city since 9/11.
The FBI said that Saipov, a resident of New Jersey, was inspired by the ISIS to carry out the terrorist attack.
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He "requested to display ISIS's flag" in his hospital room and stated that he felt good about what he had done, the complaint said.
Saipov told investigators he was inspired by Islamic State videos, in particular one showing ISIS leader Abu Bakr al-Baghdadi, it said.
The suspect decided to conduct a truck attack "to inflict maximum damage against civilians" and that he specifically chose to strike on Halloween "because he believed there would be more civilians on the street for the holiday," it said.
He planned to use the truck to hit pedestrians in the area where he carried out the attack and "then proceed to the Brooklyn Bridge to continue to strike pedestrians," the complaint said.
"Saipov wanted to kill as many people as he could," it said.
"In particular, Saipov was motivated to commit the attack after viewing a video in which Abu Bakr al-Baghdadi... questioned what Muslims in the US and elsewhere were doing to respond to the killing of Muslims in Iraq," it said.
During a search operation, federal officials obtained two cell phones from his possession. One cellphone contained approximately 90 videos, many of which appear to be, ISIS- related propaganda.
The cell phone also contained approximately 3,800 images, many of which appear to be ISIS propaganda.
Internet search history of the second cell phone reflects, among other things, that Saipov searched for a store in Passaic in New jersey on October 4, searched for Halloween in New York on October 15, and searched for truck rentals on October 18, the complaint said.
In carrying out the attack, Saipov relied on the playbook laid out by ISIS in recent years, officials said.
"He appears to have followed almost exactly to a 'T' the instructions that ISIS has put out in its social media channels before, with instructions to their followers on how to carry out such an attack," John Miller, New York Police Department commissioner for intelligence and counter- terrorism, said at an earlier news conference.
A handwritten "document" found near the scene had both Arabic and English text, and included the message that the Islamic State would endure, the complaint said.
Inside the Bag, law enforcement officers recovered, among other things, three knives and a wallet.
Hours after Saipov's court appearance, President Donald Trump said he should be executed.