For years, Germany has been strengthening security measures around Christmas markets in reaction to an attack in 2016 in which a man killed 12 people by driving a truck into a market in central Berlin.

The police have deployed more uniformed and plainclothes officers. They have erected barriers to keep cars and large vehicles away from crowds, set up security cameras and cordoned off large pedestrian-only areas. In some markets, they have enforced stop-and-search protocols. And intelligence services have stepped up their surveillance, often working closely with migration authorities.

Germany also banned knives at Christmas markets this year, after a deadly knife attack at a festival in August. Offenders now face a fine of up to 10,000 euros (more than $10,400).

Yet an attacker still managed to pierce that vigilance on Friday by driving into a crowd at a Christmas market in the eastern city of Magdeburg, killing at least five people, including a 9-year-old child. The attacker entered the market through an emergency exit before leaving through another one, according to the Magdeburg police. The driver, who the authorities identified only as Taleb A. in keeping with Germany’s privacy laws, was arrested shortly afterward.

“This is going to be a big debate in Germany,” said Peter Neumann, a professor of security studies at King’s College London. “The fact that that was possible yesterday shows that something must not have worked.”

Outdoor Christmas markets are a beloved German tradition and tourist attraction, but the large crowds they attract have made them targets.