The German authorities presented the bare outlines of a terrorism plot that they said involved at least one person trained at a militant camp in Afghanistan or Pakistan and a cache of material for producing explosives. The men had been under surveillance for seven months, but the authorities said they decided to move fast when the three began preparations for testing an explosive device.
“We succeeded in preventing a concrete and imminent danger,” the interior minister, Hans-Peter Friedrich, said in a statement that acknowledged assistance from foreign investigators. “This proves that Germany continues to be in the cross hairs of international terrorists, and we need to remain vigilant.”
The authorities said more information would be released at the federal prosecutor’s headquarters in Karlsruhe on Saturday, when the men would be brought before a judge. But some details emerged from law enforcement officials and the German news media, which said at least two of the men were Moroccan, one with German citizenship and the other living in Germany illegally. The third was said to be a German of Iranian descent, though some reports said he was of Moroccan descent.
It was not at clear if there was any connection between the arrests and the terrorist attack in Morocco on Thursday, when at least 16 people were killed after a bomb was detonated in a packed cafe in the popular tourist city of Marrakesh. But experts said it was likely that Moroccan and German intelligence services had recently cooperated.
The German news media reported that the three suspects had been taken into custody in the western state of North Rhine-Westphalia and that the authorities had confiscated materials for making explosives, including acetone, a volatile substance that terrorists often use. Officials identified the suspects only as Abdeladim K., Jamil S. and Ahmed Sh. German authorities do not typically release the names of criminal suspects.
According to the newspaper Bild, one of the men was from Düsseldorf and the other two were from nearby cities, Essen and Bochum. All three were arrested at 6:30 a.m. in raids in Düsseldorf and Bochum.
The target was uncertain. Bild, Germany’s most widely read and generally reliable newspaper, reported that the terrorist cell might have planned to hit the popular Eurovision Song Contest on May 14, though that event’s organizers said they had not been alerted to any such threat.
The newspaper Die Welt anonymously quoted an investigator as saying that the suspects were planning to attack public transportation in a large German city. An American official who was briefed on the operation said the plot was aimed at buses or depots.
The arrests and accusations, which combined elements of homegrown terrorism with concerns about the lawless Afghanistan-Pakistan border region, shook Germany’s sense of calm. The country was put on edge last year as well after a terrorism alert involving what the authorities said was concrete information of a planned attack.
The increased security measures established at public areas like transportation hubs had eased in recent months, but law enforcement had quietly continued to pursue leads on the threats aggressively, including a suggestion that terrorists planned to strike the Reichstag, home to the lower house of Parliament.
Wolfgang Bosbach, a member of the lower house and chairman of the Committee on Internal Affairs, said investigators had closely monitored travel to the Afghan border area and noted “a significant number of returnees.”
“The recent arrests must not surprise anyone who deals seriously with the matter,” Mr. Bosbach said.
The Rhine-Main region has a large population with ties to North Africa and is one of Germany’s centers of the Salafist movement, a radical fundamentalist school of Islamic thinking, along with Hamburg, Berlin and the region around Frankfurt, according to Guido Steinberg, an expert on terrorism with the German Institute for International and Security Affairs.
“We do not know yet exactly how big this is,” Mr. Steinberg said. “But that something could happen this year is anything but absurd. The Salafist scene has grown significantly in Germany in recent years. The number of converts has increased substantially as well.”
According to the official Germany News agency, the three men had planned to test an explosive on Thursday night, but delayed for some reason. The authorities said the men were arrested without previously issued warrants, perhaps indicating a decision to move quickly.
Souad Mekhennet contributed reporting from Marrakesh, Morocco; Jack Ewing from Frankfurt; and Eric Schmitt from Washington.