New York police officials say the attacker appears to have radicalized himself online and that it does not appear anyone else was involved, though they said that continues to be a key question in the international investigation launched after the Halloween attack in Lower Manhattan killed eight people and wounded a dozen others.

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Federal authorities charged Sayfullo Saipov, the suspected attacker, with providing support to a terrorist organization, saying that he was inspired by the Islamic State to carry out the rampage. The militant group, also known as ISIS, has urged its supporters to use vehicles for attacks.

In the charging document, filed Wednesday, authorities said Saipov planned for a year to carry out an attack in the United States and ultimately chose Halloween because he believed more people would be outside as potential targets.

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The federal prosecution against Saipov was just hours old when a potentially complicating factor emerged in the form of a presidential tweet. Since the attack, Trump has publicly criticized the American criminal justice system and weighed sending Saipov to the U.S. military prison at Guantanamo Bay.

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In messages posted on Twitter late Wednesday and early Thursday, Trump twice called for Saipov to get the death penalty, while also abandoning the Guantanamo Bay idea.

“Would love to send the NYC terrorist to Guantanamo but statistically that process takes much longer than going through the Federal system,” Trump wrote early Thursday. He continued: “There is also something appropriate about keeping him in the home of the horrible crime he committed. Should move fast. DEATH PENALTY!”

Trump’s comments, much like remarks he made about Army Sgt. Bowe Bergdahl, could create a hurdle in the federal case. While one of the charges against Saipov — one count of violence and destruction of a motor vehicle — could carry with it a possible death sentence, the Justice Department has not yet said whether it will seek that penalty. If prosecutors do pursue a rare federal death sentence against Saipov, defense attorneys could argue that Trump’s tweets may prevent a jury from giving the suspect a fair trial.

The remarks from Trump broke from the tradition that presidents and other senior officials refrain from commenting on ongoing cases in ways that could complicate proceedings, though he is not the first commander in chief to do so. In 2009, then-President Barack Obama weighed in on the case against Khalid Sheikh Mohammed, self-proclaimed mastermind of the Sept. 11, 2001, attacks and suggested he would get the death penalty; Obama then backtracked and said he did not mean to prejudge the case.

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Saipov was charged in a criminal complaint, though authorities will eventually ask a grand jury to indict him, and he could possibly face additional charges. His next court appearance is technically scheduled for November 15, though that is very likely to change after an indictment is filed.

David Patton, Saipov’s attorney, declined to comment Thursday on Trump’s comments. In a statement released before Trump tweeted, Patton said: “In a case like this involving so much tragedy, it’s more important than ever to let the judicial process play out. How we as a society treat Mr. Saipov will say more about us than it will about him.”

At a speech Thursday in New York City that was scheduled before the truck attack, Attorney General Jeff Sessions highlighted the work federal prosecutors have done bringing cases against terrorism suspects in federal court.

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He noted particularly the recent conviction of Ahmad Khan Rahimi, who set off bombs in New York and New Jersey last year; the recent unsealing of charges against three men who plotted to bomb the New York City subway and Times Square; and the apprehension of Mustafa al-Imam, a Libyan national charged with participating in the 2012 Benghazi attacks.

The remarks, in some ways, seemed to be a subtle hint to the president that terror suspects can face justice in American courts. But Sessions, a vocal supporter of using the military prison at Guantanamo Bay, also made mention of the facility there.

“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, according to a prepared copy of his remarks. “If anyone has any doubt about that, they can ask the more than 500 criminals whom the Department of Justice has convicted of terrorism-related offenses since 9/11. And they can ask the dozens of enemy combatants in Guantanamo Bay.”

Sessions, who attended a roll call Thursday with officers, also heaped praise on the New York City Police Department, in particular the officer who shot and wounded Saipov.

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The attorney general has had a strained relationship with New York City’s leaders, and in April declared that gang murders there were the “predictable consequence of the city’s ‘soft on crime’ stance.” Sessions’s remarks drew pushback from New York officials, as have some of Trump’s comments.

By Thursday afternoon, Trump had returned his focus to immigration, saying that he was calling for Congress to end a visa lottery program the suspected attacker used to get into the country years earlier.

Trump, in one of his tweets about the New York attack, cited one of the most incendiary parts of the criminal complaint filed against Saipov. Authorities said that Saipov told them he felt good about what he had done and, while speaking to investigators, “requested to display ISIS’s flag in his hospital room.”

In the criminal complaint, the FBI described what Saipov said in his Manhattan hospital room, depicting him as a man who had reams of Islamic State propaganda on his phones and carefully plotted what he was doing.

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Saipov told agents he wanted to kill as many people as he could, court papers state, and he considered putting Islamic State flags at the front and back of his truck before deciding that would draw too much attention.

Authorities said Saipov told them that while he first decided a year ago to carry out an attack in the United States — the country where he moved in 2010 on a diversity visa and became a legal permanent resident — he only decided to use a truck two months before.

Saipov rented one on the week before the attack to practice making turns with it, authorities said. A neighbor said he thought it was suspicious that Saipov was driving an apparently empty truck in recent weeks near their homes in New Jersey.

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Police say that on Tuesday afternoon, Saipov drove a truck onto the bike path along the west side of Manhattan and targeted cyclists and pedestrians as he careened south. Among those Saipov is accused of killing were a group of childhood friends from Argentina, now in their late 40s, who had been planning a trip to New York for years; a young mother; and two men in their 20s and 30s from New York and New Jersey.

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Saipov told authorities he was particularly inspired by a video capturing Islamic State leader Abu Bakr al-Baghdadi asking Muslims in the United States what they were doing to respond to the killing of other members of their faith in Iraq, the complaint states.

Officials have said that Saipov apparently became radicalized online after he came to the United States. He “appears to have followed almost exactly to a T the instructions that ISIS has put out in its social media channels” laying out guidance for carrying out an attack, according to John Miller, the deputy commissioner of intelligence and counterterrorism at the New York City Police Department.

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Among other things, officials say Saipov used a rented truck, had brought knives and a stun gun as additional weapons and left behind notes declaring his allegiance. According to court papers, one note, written in Arabic, could be translated in part to read: “Islamic Supplication. It will endure.”

As the Islamic State has suffered battlefield losses and seen its self-declared caliphate shrink, terrorism by vehicle has become the attack of choice for the group’s adherents and supporters in other areas. The tactic has been used, with deadly results, in France, Britain, Germany, Sweden, Spain and Canada.

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Investigators are still exploring whether anyone else had any knowledge of or aided in the New York plot. The FBI said briefly on Wednesday it was seeking another man — 32-year-old Mukhammadzoir Kadirov, or Muhammad Kadirov — in connection with the investigation. The bureau gave no indication why they were seeking him and, minutes later, reversed course, saying they had found him but providing no further details.

A person who was in touch with both Saipov’s and Kadirov’s families on Wednesday said that Kadirov is in New Jersey, has retained an attorney and is cooperating with law enforcement officials, but that he was not under arrest as of Wednesday evening. The person, who spoke on the condition of anonymity because of the sensitivity of the subject, said Kadirov is Saipov’s cousin and seemed “utterly shocked and horrified” by what Saipov had done.

The rampage on Tuesday afternoon ended when Saipov crashed into a school bus and emerged from his truck armed with a paintball gun and pellet gun, police said. A passer-by flagged down police officers responding to an unrelated call at a school in the area, and one of them shot and wounded Saipov, police said.

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Authorities also said Saipov intended to continue his attack beyond the bike path. He told investigators he intended to keep going to the Brooklyn Bridge to kill even more people, the complaint states, but was apparently unable to after crashing the truck.

Eli Rosenberg and Abigail Hauslohner in Paterson, N.J.; Renae Merle in New York; and Devlin Barrett, Sari Horwitz, Julie Tate, Philip Rucker, Amy B Wang and Samantha Schmidt in Washington contributed to this report, which has been updated.