The interior minister of North Rhine-Westphalia said officials had been tipped off about an "Islamist-motivated threat" ahead of the Jewish holiday.
A 16-year-old boy and three other people have been arrested in Germany over an alleged Islamist plot to attack a synagogue on the Jewish holiday of Yom Kippur.
Police cordoned off the synagogue in the city of Hagen, south of Dortmund, on Wednesday and an evening worship service was called off.
Herbert Reul, interior minister of North Rhine-Westphalia, said officials had received "very serious and concrete information" that there could be an attack on the synagogue during Yom Kippur.
The tip, he said, had pointed to "an Islamist-motivated threat situation" and named both the possible timing and would-be perpetrators.
The teenager, a Syrian national who lives in Hagen, was detained on Thursday morning while the three others were arrested after a raid on an apartment.
Police are still investigating whether they were involved in the alleged plan. Reul added that other searches were ongoing in Hagen, but gave no details.
The detentions took place on Yom Kippur, the holiest day in Judaism, and two years after a deadly attack in another German city on the Yom Kippur holiday.
Police were on the scene on Thursday with sniffer dogs, which found no trace of explosives. Dortmund police spokesman Gunnar Wortmann told AP: "We had information that there was a threat to a Jewish institution here in Hagen.
"Of course, we immediately adjusted the police protection measures in this regard and are also in close contact with the local Jewish community."
Alleged plot comes two years after deadly Yom Kippur assault
Two years ago on Yom Kippur, German far-right extremist Stephan Balliet attacked a synagogue in the eastern German city of Halle, killing two people.
Balliet had posted an anti-Semitic diatribe online before making his way to the place of worship, where 52 people had gathered to mark Yom Kippur.
After failing to get through the doors, he shot and killed a 40-year-old woman in the street outside and a 20-year-old man in a nearby kebab shop.
The incident was considered one of the worst anti-Semitic assaults in the country's post-war history. In December 2020 Balliet was jailed for life for the attempted murder of 68 people, Holocaust denial and incitement.
German Justice Minister Christine Lambrecht sharply condemned the foiled Hagen attack on Thursday. “It is intolerable that Jews are again exposed to such a horrible threat," she said, "and that they cannot celebrate the start of their highest holiday, Yom Kippur, together."