The main suspect in the New York truck rampage that killed eight people on Tuesday has told investigators he began planning the attack a year ago.
Sayfullo Saipov, 29, said he intended to kill as many people as possible and "felt good" about the Halloween attack.
The Uzbek immigrant, who faces federal terrorism charges, says he was inspired by so-called Islamic State. He was shot and injured by police at the scene.
US President Donald Trump tweeted that he should receive the death penalty.
The charges filed against Mr Saipov are federal, which means the government can override New York state's ban on capital punishment.
Mr Saipov appeared in court in a wheelchair 24 hours after mowing down cyclists and pedestrians on a bike lane in Lower Manhattan on Tuesday.
Six people died at the scene and two more in hospital. Twelve people were injured, four of whom remain in critical condition.
Prosecutors say Mr Saipov spoke freely to them, waiving his right to avoid self-incrimination while in custody.
Mr Saipov was charged with one count of providing material support and resources to IS and another count of violence and destruction of motor vehicles.
New York Police's Deputy Commissioner John Miller said the suspect appeared "to have followed almost exactly to a 'T'" IS instructions on how to carry out such an attack.
The FBI says they have located a second Uzbek man, 32-year-old Mukhammadzoir Kadirov, who was wanted for questioning in connection with the attack.
Sayfullo Saipov had reportedly lived in Tampa, Florida, before moving to Paterson, New Jersey.
He arrived in America from Uzbekistan in 2010 and is a legal resident of the country. Uber confirmed he had been working as a driver for them.
CBS News quotes an intelligence source as saying he was known to US authorities after his name was associated with the subjects of FBI counter-terrorism investigations in 2015.
The source says he had some contact with individuals who were considered radicalised extremists, at least one of whom was Uzbek. It is unclear whether those being investigated were in the US or overseas.
It is not known if Mr Saipov, who was not the main focus of the investigation, was interviewed at that time by the FBI.
New York has increased the police presence at key transport hubs, and there will be additional uniformed and plain-clothes police on duty at Sunday's New York Marathon.
President Trump said he was taking steps to end the diversity lottery programme - the immigration system under which the suspect entered the country.
He also attacked the US justice system, and said he would consider sending the suspect to the US military prison at Guantanamo Bay, Cuba.
The administration says it has not ruled out adding Uzbekistan to a list of countries in the president's proposed travel ban.
The mayor of New York City and the governor of New York state have both praised the resilience of New Yorkers - and urged people not to "politicise" a tragedy that took eight lives.
Governor Andrew Cuomo and Mayor Bill de Blasio also suggested New York's strict gun control laws had ensured that high-powered weapons could not get into the hands of terrorists.
The attacker rented the truck from a New Jersey branch of retailer Home Depot on Tuesday afternoon before driving to New York City and entering the bike lane, police said.
Video cameras show the van driving at very high speed, appearing to target bike riders and pedestrians.
After the van collided with a school bus, the van driver emerged and brandished what appeared to be two weapons.
Mr Saipov was shot and wounded at the scene by an NYPD officer who was alerted to the attack by witnesses.
It was the deadliest terror attack in the city since the 9/11 attacks that killed nearly 3,000 people in 2001.
A group of five Argentine friends celebrating the 30th anniversary of their college graduation were among the dead.
The men - all aged 48 or 49 - were named as Hernán Diego Mendoza, Diego Enrique Angelini, Alejandro Damián Pagnucco, Ariel Erlij and Hernán Ferrucci.
Belgian officials said Anne-Laure Decadt, a 31-year-old from Staden in Flanders, was also killed. Three Belgians were wounded.
Two other victims, both Americans, have been identified as Darren Drake, 32, and Nicholas Cleves, 23.