Police observe the half marathon in front of the Berlin Cathedral, in Berlin, Sunday, April 8, 2018. The German daily Die Welt is reporting that police have foiled a knife attack on a half-marathon in Berlin. The paper says special police forces detained four men in connection with Sunday's sports event. They said the main suspect was planning to kill participants and spectators with knives. (Christophe Gateau/dpa via AP)

Police observe the half marathon in front of the Berlin Cathedral, in Berlin, Sunday, April 8, 2018. The German daily Die Welt is reporting that police have foiled a knife attack on a half-marathon in Berlin. The paper says special police forces detained four men in connection with Sunday's sports event. They said the main suspect was planning to kill participants and spectators with knives. (Christophe Gateau/dpa via AP)

BERLIN (AP) — Six people were detained in connection with what police and prosecutors allege was a plan to carry out an attack on Berlin’s half-marathon Sunday, German authorities said. A police spokesman later said that no athletes or spectators had been in danger.

“There were isolated indications that those arrested, aged between 18 and 21 years, were participating in the preparation of a crime in connection with this event,” prosecutors and police wrote in a joint statement.

Berlin police tweeted that six people were detained in cooperation with the city’s prosecutor’s office.

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The German daily Die Welt first reported that police foiled a plot to attack race spectators and participants with knives.

The main suspect allegedly knew Anis Amri, a Tunisian who killed 12 people and injured dozens more when he drove a truck into a Christmas market in Berlin in December 2016, Die Welt reported.

Police spokesman Thomas Neuendorf told local broadcaster RBB that the evidence of a viable and imminent attack was not yet confirmed.

“They were very vague indications, but because of the high threat level we immediately started with our police searches and arrests,” Neuendorf said.

“But at the moment there are no really concrete indications that an attack on the Berlin half-marathon was foreseen,” he said.

One of the apartments Berlin special forces police raided before the race started Sunday was also searched after the Christmas market attack, Die Welt reported.

The newspaper also said the main suspect, who was not identified, had prepared two knives. It wrote that in one of the searched apartments, dogs trained to find explosives barked when they were taken into the dwelling’s basement.

Mobile phones and computers were among the confiscated evidence being examined, Neuendorf said.

The police spokesman added that there had been “no danger at any time for the runners, participants and staff” during the race, which organizers said attracted some 250,000 spectators.

Neuendorf told German news agency dpa that the investigations were centered on people “whom we regard are associated in the Islamist-terrorist area.”

A record 36,000 athletes entered the 21.1-kilometer (13.1-mile) race, which was won by Erick Kiptanui of Kenya in a course-record 58 minutes and 42 minutes.

The half-marathon was being guarded by some 630 police officers, dpa reported.

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The local daily Tagesspiegel reported that the primary suspect had been under observation for two weeks around the clock.

After a foreign intelligence service tipped off German authorities that he was planning to attack the half-marathon, police raided apartments and two vehicles in the Charlottenburg and Neukoelln districts of the city, Die Welt said.