Two boys aged 15 and 16 have been arrested by German authorities on suspicion of planning a militant Islamist attack on a Christmas market or a synagogue.
The younger boy, a German-Afghan from the western town of Burscheid, was detained after his home was searched.
The seriousness of the alleged plot was not immediately clear to authorities.
Police apparently intervened to be on the safe side, because the boy had identified a time and place.
"It seemed very concrete," said local interior minister Herbert Reul.
"We received a tip from abroad about possible plans for an attack involving someone from North Rhine-Westphalia, and that's when the security apparatus kicked in."
The 16-year-old, described as a Russian national, was detained in the town of Wittstock/Dosse in the north-eastern state of Brandenburg outside Berlin on suspicion of preparing the alleged plot.
Public prosecutors in the two states will now take over the investigation.
Mr Reul said a specific Christmas market had been agreed by the pair as a target and public broadcaster WDR reported that Friday 1 December had been decided as the date.
What is unclear is where the alleged plot was supposed to unfold. Initial reports referred to Cologne, to the south-west of Burscheid, but German sources later suggested it was the nearby city of Leverkusen.
The pair are said to have exchanged information over the Telegram messaging app about using home-made incendiary devices or a van to carry out their plans.
The alleged plot on a Christmas market brought back memories of the December 2016 attack in Berlin, when a jihadist hijacked a lorry and drove it into a crowd on Breitscheidplatz, causing the deaths of 13 people.
The arrests emerged as German domestic intelligence chief Thomas Haldenwang warned that the risk of Islamist militant attacks was "real and higher than it has been for a long time" because of the Israel-Hamas war.
Mr Haldenwang, who is head of the Federal Office for the Protection of the Constitution (BfV), cited a variety of groups spreading hatred, incitement and antisemitism, adding that he had repeatedly stressed than an attack could happen in Germany any day.
Last month, the BfV said it was working to outlaw support for Hamas and a pro-Palestinian network called Samidoun. However, Mr Haldenwang said it was jihadist groups such as al-Qaeda and so-called Islamic State that were exploiting the situation to convey a victim narrative to Muslims in the West.
He warned that the potential for attacks on Jewish and Israeli people and institutions, and the West as a whole, had significantly increased.
