US President Donald Trump has ordered the Department of Homeland Security to "step up" vetting measures, after a deadly truck attack in New York.
Eight people were killed and 11 injured when the driver of the truck hit people on a cycle path in Lower Manhattan. Five Argentines were among the dead.
A 29-year-old man was shot by police as he left the car and detained. Media named him as Sayfullo Saipov, an Uzbek immigrant who came to the US in 2010.
Officials said it was a terror attack.
The suspect is now in a hospital. A note was found in the truck that referred to so-called Islamic State, law enforcement sources were quoted as saying by US media.
Sayfullo Saipov had been arrested in Missouri last year over a traffic fine.
How did US authorities respond?
President Trump had been briefed about what happened in New York on Tuesday afternoon, the White House said.
He then took to Twitter, saying:
"In NYC, looks like another attack by a very sick and deranged person. Law enforcement is following this closely. NOT IN THE U.S.A.!"
"We must not allow ISIS [Islamic State] to return, or enter, our country after defeating them in the Middle East and elsewhere. Enough!"
"My thoughts, condolences and prayers to the victims and families of the New York City terrorist attack. God and your country are with you!"
He also announced that he had ordered the Extreme Vetting Program to be stepped up. He did not elaborate further.
Meanwhile, New York City Mayor Bill de Blasio said the incident in Manhattan was a "cowardly act of terror aimed at innocent civilians".
"We know that this action was intended to break our spirit. But we also know that New Yorkers are strong, New Yorkers are resilient and our spirit will never be moved by an act of violence and an act meant to intimidate us," he said.
Police urged people to avoid the usually busy area as a major emergency services operation got under way.
How did the attack unfold?
At a news briefing, New York Police Department (NYPD) Commissioner James O'Neill described what had occurred:
Shortly after 15:00 local time (19:00 GMT), a white pick-up truck rented from retailer Home Depot struck cyclists and pedestrians while being driven along the West St-Houston St path for many blocks
The vehicle then hit a school bus, injuring two adults and two children on board, and came to a halt
The driver emerged holding what appeared to be two handguns and made a statement "consistent with a terror attack"
He was shot in the abdomen by a police officer stationed near the scene
A paintball gun and a pellet gun were recovered from the scene
The suspect reportedly shouted "Allahu Akbar" ("God is great") as he left the truck.
He was then shot and injured in the stomach by a police officer, the NYPD said.
Mangled bicycles littered the scene of the attack, which occurred as much of the city was celebrating Halloween.
In a statement, the Argentine foreign ministry said five of those killed were from Argentina.
It said the five men - all from the city of Rosario - were part of a group of friends celebrating the 30th anniversary of their graduation from a polytechnic school.
Another member of the group was in hospital.
One witness, identified as Eugene, told ABC Channel 7 that he saw the white pick-up truck driving fast down the cycle path alongside the West Side Highway, near Stuyvesant High School, at full speed and hitting a number of people.
He also reported hearing about nine or 10 shots.
Another witness, who gave his name as Frank, told local TV network NY1 that he had seen a man running around an intersection and heard five to six gunshots.
"I saw he had something in his hand, but I couldn't tell what it was. But they said that it was a gun...
"When the cops shot him, everybody started running away and it got a little bit crazy right there. So when I tried to look again, the guy was already down."
What is known about the suspect?
The NYPD only identified the attacker as a 29-year-old male,
However, US media named him as Sayfullo Saipov, who had reportedly lived in Tampa, Florida, and Paterson, New Jersey.
He arrived to America from Uzbekistan in 2010 and is believed to be a legal resident in the country.
Three officials said Mr Saipov had previously come to the federal authorities' attention as a result of an unrelated investigation, the New York Times reported.