By Nicole Chavez and Eric Levenson, CNN

    (CNN) -- The man accused of using a truck to plow down pedestrians and cyclists on a New York bike path and killing eight people was so devoted to ISIS that he wanted to display the terror group's flag in his hospital room, documents show.

Details that emerged at Sayfullo Saipov's initial court appearance Wednesday paint a picture of a man who reportedly had planned even more carnage after the deadly attack on a bike path near the World Trade Center in Lower Manhattan.

The 29-year-old Uzbek native had planned to continue striking pedestrians on the Brooklyn Bridge on Tuesday, he told investigators, according to a criminal complaint.

Instead, Saipov crashed his truck into a school bus in Lower Manhattan and then jumped out, brandishing imitation firearms, officials said. New York police Officer Ryan Nash fired nine shots at him, wounding him in the abdomen, before police took him into custody.

Saipov did not say much when he appeared in court Wednesday using a wheelchair.

But he waived his right to silence and told investigators that he had planned Tuesday's attack for about a year, according to the criminal complaint.

Read the federal complaint against the suspectSaipov picked Halloween to carry out the deadliest attack in New York since 9/11 because he believed more people would be out on the streets for the holiday, according to the criminal complaint. More than a dozen people were also injured Tuesday.

He was inspired to carry out the attack after watching ISIS videos and closely followed the ISIS playbook for an attack, according to the complaint.

Saipov had about 90 videos and 3,800 images on a cell phone featuring ISIS propaganda, including video of a beheading, according to the complaint.

Saipov is charged with providing material support to ISIS, violence and destruction of motor vehicles, said Joon H. Kim, acting US attorney for the Southern District of New York.

Saipov didn't enter a plea Wednesday to the federal terror charges, a source at the US attorney's office told CNN.

 On Twitter, President Donald Trump said Saipov should be executed and added that he "would love" to send him to the Guantanamo Bay detention camp in Cuba.

"NYC terrorist was happy as he asked to hang ISIS flag in his hospital room. He killed 8 people, badly injured 12. SHOULD GET DEATH PENALTY!' the President tweeted Wednesday night.

Presidents typically do not comment on pending criminal cases because their statements could be seen as influencing a potential jury pool.

In prepared remarks Thursday, US Attorney General Jeff Sessions did not rule out using Guantanamo Bay to prosecute terrorists. The remarks did not specifically refer to the Saipov case.

"Terrorists should know: This administration will use all lawful tools at our disposal, including prosecution in Article III courts and at Guantanamo Bay," Sessions said.

New York Mayor Bill de Blasio said Thursday that he wasn't clear about all the legalities but said he did not believe in the death penalty in general.

"I believe this is a person who should rot in prison for the rest of his life," de Blasio said.

After his arrest, Saipov told investigators who talked to him at Bellevue Hospital that he planned the attack for about a year. Two months ago, he said, he decided to use a truck "to inflict maximum damage against civilians," the complaint said.

On October 22, he rented a truck from a Passaic, New Jersey, store that authorities earlier had identified as Home Depot to practice making turns ahead of the attack, according to the complaint.

Carlos Batista, a neighbor of Saipov in Paterson, New Jersey, told CNN that he'd seen the same type of truck around the neighborhood "for the last couple of weeks."

According to the complaint, Saipov went to the same store and rented the truck for two hours. He "had no intention of ever returning it," the document 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," it said.

"Saipov wanted to kill as many people as he could," the complaint said.

Investigators also sought to question Mukhammadzoir Kadirov, 32, an Uzbek national, in relation to the attack. Law enforcement sources said Wednesday night they found the man and are questioning him.

"We have found him, and we'll leave it at that," said William Sweeney, assistant director in charge of the FBI's New York field office.

The state of New York began installing concrete barriers to "enhance protections" for the bike path where the attack occurred.

The work began Thursday as part of a statewide review of shared-use paths, according to a statement from the New York State Department of Transportation.

During an interview with investigators, Saipov asked to hang an ISIS flag in his hospital room and said "he felt good about what he had done," the complaint said.

At some point, Saipov had considered displaying ISIS flags in the front or back of the truck he used during the attack but decided against it to avoid drawing attention to himself, he told investigators.

Authorities did not find any flags near the truck, but they found a handwritten note in Arabic expressing affinity for ISIS.

"The gist of the note was the Islamic State would endure forever," said John Miller, deputy commissioner for intelligence and counterterrorism for the New York police.

ISIS has not made any claim of responsibility for the attack, but Saipov told investigators that the group's videos inspired him, in particular one that shows ISIS leader Abu Bakr al-Baghdadi questioning "what Muslims in the United States and elsewhere were doing to respond to the killing of Muslims in Iraq," the complaint said.

The videos that law enforcement found on Saipov's cell phone include graphic scenes of ISIS fighters killing prisoners, beheadings and "instructions for how to make a homemade improvised explosive device," the complaint said.

Saipov has also been linked to social media accounts that contain ISIS-related material, a law enforcement official said.

The tactic of turning an ordinary vehicle into a lethal weapon is familiar to groups such as ISIS. In 2014, an ISIS spokesman called for attacks using improvised weapons such as knives, rocks, poison and cars.

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