A woman lays flowers near the scene where a van plowed into a crowd of pedestrians in north London on Monday, June 19. A man was arrested, police said, and is being held on suspicion of terrorism offenses. The attack happened near a mosque, and British Prime Minister Theresa May said it was directed at Muslims. TOLGA AKMEN/AFP/Getty Images

Police officers stand at a cordon near the scene. The incident happened on Seven Sisters Road, a busy thoroughfare in Finsbury Park. DANIEL LEAL-OLIVAS/AFP/Getty Images

Forensics officers work at the scene. TOLGA AKMEN/AFP/Getty Images

Pedestrians watch investigators at work. Wheatley/WENN/Newscom

A forensics officer examines the interior of a van. Carl Court/Getty Images

Area residents react at the scene. TOLGA AKMEN/AFP/Getty Images

A forensics officer stands next to a van near the Finsbury Park Mosque. Carl Court/Getty Images

Investigators work at the scene. DANIEL LEAL-OLIVAS/AFP/Getty Images

Police guard a street in the Finsbury Park area. DANIEL LEAL-OLIVAS/AFP/Getty Images

Police officers gather near a van on Whadcoat Road. James Gourley/REX/Shutterstock/AP

Locals pray near the mosque. Yui Mok/AP

A police officer at the scene of the incident. James Gourley/REX/Shutterstock/AP

Police help a woman at the scene. Courtesy Cynthia Vanzella

London's Metropolitan Police said officers were called just after midnight. Thomas Van Hulle/Twitter

Police and ambulance crews gather at the scene. REX/Shutterstock/AP

Injured pedestrians are treated by emergency workers. James Gourley/REX/Shutterstock

"People were shouting, 'This is an act of terrorism!'" witness Hillary Briffa said.

Courtesy Cynthia Vanzella

Resident Cynthia Vanzella said she was in bed when she heard people shouting. She went to the window and saw "loads of people gathering" in a corner across the road. Courtesy Cynthia Vanzella

Police officers stand guard. Victoria Jones/AP

Emergency crews move one of the injured. James Gourley/REX/Shutterstock/AP