Tipped off by a foreign intelligence agency, they believed at least one of them had a link to Anis Amri, a Tunisian who barreled a truck through a Berlin Christmas market in the last Islamic terrorist attack on German soil, according to the Associated Press. Police were watching one suspect's movements around the clock.

On Sunday police said they arrested the man and five co-conspirators and foiled an alleged plot intended to inflict horror. The arrested men had planned to wade through dense crowds of runners and spectators at the Berlin Half Marathon, using knives to slash at anyone in reach, authorities said. Police have not released the names of the men who have been detained or any information about the charges they face.

More than 36,000 people participated in the event, setting an attendance record, according to the BBC. The race was guarded by more than 630 police officers and went off undisturbed.

“In the run-up to the Berlin Half Marathon, there were isolated indications that the six detainees, between the ages of 18 and 21, may have been involved in the preparation of a crime in connection with this event,” the Berlin police department said in a statement on its website.

Police said they were also on alert after a man plowed a van into a crowd  in the northern German city of Münster on Saturday, killing two and injuring 20 others.

Investigators have not released a motive, but said the man was a “psychologically disturbed” German national and that they don't suspect a link to terrorism, according to The Washington Post's Souad Mekhennet and Michael Birnbaum. The man had had several run-ins with local police, including a time when he threatened his father with an ax.

Still, “the attack sparked fears on a continent that has been hit repeatedly by attacks in which vehicles have been turned into weapons in crowded city centers,” Mekhennet and Birnbaum reported.

In Berlin, police conducted raids before the race started, including at a house that was searched after the Christmas market attack, CBS News reported. In one of the homes, search dogs indicated the presence of explosives, although police haven't released more details.

On Sunday, German Interior Minister Horst Seehofer said the government would do everything it can to protect citizens. “We have again experienced that absolute security is unfortunately not possible,” he said, according to CBS News.

Still, the German anti-terror apparatus has been bolstered in the past two years.

According to Reuters, after Amri's attack, German leaders called for tougher security measures. No major Islamist attack has been carried out in the country since then.

The Berlin Half Marathon has been held annually in Germany's capital since the early 1980s and now attracts tens of thousands of runners from around the world. Erick Kiptanui, of Kenya, won Sunday's race in 58 minutes, 42 seconds, a course record that equaled the fastest time in the world this year, race organizers said on their website.

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