Three militant Islamic terror suspects had acquired vans that might have been intended for use in bomb attacks on U.S. and other targets in Germany, a spokeswoman for prosecutors said Saturday.
Estimates of the size of the plot ranged as high as 50 people, and prosecutors were looking the possibility of a wider terrorist network.
German authorities think the two German converts and a Turk arrested Tuesday were only the leading tip of the planning, said Petra Kneuer, spokeswoman for the Federal Prosecutor's Office.
She said authorities were investigating seven more suspects inside and outside Germany as part of what could be a larger network. Reports that about 50 people are believed to be involved overall are "not unrealistic," she added.
The purpose of the vans has yet to be nailed down, Kneuer said when she was asked about a report in the news weekly Focus that the three suspects had obtained the vehicles across the border in France.
"So far it hasn't been shown that there really was a link to the planned attacks," she told The Associated Press.
Joerg Ziercke, the head of the Federal Crime Office, said the arrests had disrupted the plan. "The peak of the danger has passed," he said.
Police said the suspects had stockpiled enough hydrogen peroxide to build bombs more powerful than those that killed 191 commuters in Madrid in 2004 and 52 in London in 2005.
They have been identified as Fritz Martin Gelowicz, described as the leader of an Islamic Jihad Union terror cell in Germany, and Daniel Martin Schneider, 22. The third has been identified as Adem Yilmaz, 29, born in Turkey but living in Germany.
All three attended terror training camps in Pakistan last year, officials said.
Last New Year's Eve one of the suspects was spotted studying a U.S. Army base at Hanau, near Frankfurt, with what security officials suspected was the intent of scouting it as a target.
Prosecutors have said potential targets might have included restaurants, pubs, discotheques, airports and other places frequented by Americans.
The three suspects allegedly belonged to the Islamic Jihad Union, an offshoot of the Islamic Movement of Uzbekistan, which has origins in that Central Asian country. The union is marked by "profound hatred" of U.S. citizens, officials said.
The extremism that fueled the alleged plot was cultivated in the city of Ulm, on the banks of the Danube River in southwestern Germany, security officials said.
Ulm, the birthplace of Albert Einstein, has been known for its cathedral, which has one of the largest church towers in the world. But in recent years it has become a hotbed of radical Islamic activity. Security officials said Gelowicz attended an Islamic center that was later closed by authorities as a security risk.
German politicians called Saturday on Islamic groups and communities in Germany to step up their commitment to the fight against terrorism. Interior Minister Wolfgang Schaeuble and Justice Minister Brigitte Zypries said Muslims should report potentially violent extremists to authorities.
"I am certain that the peace-loving Muslims in our country want to keep violent radicals from discrediting their faith community," Zypries was quoted as saying by the weekly Bild am Sonntag.