Four people were “very, very ill” and in hospital on Tuesday, a local official in Liverpool said, a day after a car drove into an enormous crowd of elated soccer fans in the city in northwest England.

Dozens more were injured in the incident on Monday evening, which the British police are not treating as terrorism. The police moved quickly to announce that they had arrested the driver of the car, a 53-year-old white man from Britain, in an apparent move to stop the spread of misinformation.

The collision ripped through what should have been a jubilant day for the city. Hundreds of thousands of people had gathered on Monday, a public holiday, to celebrate Liverpool F.C.’s victory in the Premier League, which is the top tier of British soccer.

Prime Minister Keir Starmer described the scenes as “appalling.”

Here is what we know.

What happened?

The Merseyside Police said they were contacted at about 6 p.m. on Monday local time after reports that a car had hit the crowd.

Video shared on social media shows a dark-color vehicle with a broken rear window swerving into the crowd and hitting parade-goers, leaving bodies on the ground. The video shows people rushing to aid the victims, including some who were trapped beneath the vehicle, and surrounding the vehicle once it stopped.

Video also shows witnesses attempting to stop the vehicle, with one person prying open the driver’s-side door before a man in the seat slams the door shut and accelerates into the crowd.