Three alleged Islamist terrorists, two of them Germans who had converted to Islam, planned bomb attacks on a string of German cities and the main US air base in Germany, federal prosecutors said Friday.

The prosecutors in Karlsruhe named the alleged target cities as Frankfurt, Dortmund, Dusseldorf, Stuttgart, Munich and Cologne. The US air base at Ramstein near Frankfurt was another potential target, they said.

Police, who had bugged the plotters' communications and surreptitiously confiscated the main ingredient in the explosive, arrested the men a year ago.

If the bombing had succeeded, it would have been Germany's bloodiest experience by far of Islamist terrorism.

Fritz Gelowicz, 29, Daniel Schneider, 22, both Germans, and Adem Yilmaz, 29, a Turkish national, are to be tried by a state court in Dusseldorf.

Many Germans remain shocked that the men had ordinary German upbringings, unlike the radicals who were born in Arab lands and moved to Germany to plot the September 11, 2001, suicide attacks on New York and Washington using hijacked airliners.

Police around Europe say "home-grown" terrorism has become as big a threat as that from radical immigrants.

Gelowicz, said to have led the plot, changed his religion to Islam and allegedly resolved five years ago to join the "jihad" or holy war against the West.

The precise allegations against the men have yet to be disclosed, but the indictment accuses them of membership in a terrorist organization on German soil and abroad, which are two separate charges.

Schneider was also indicted for the attempted murder of a policeman because he grabbed a gun and fired it while fleeing arrest in a small country town on September 4,
2007.

Earlier statements by the prosecutors said the group was assigned to carry out the attack by the Islamic Jihad Union, a group of Uzbek origin that has its camps in Pakistan or Afghanistan.

They bought 12 drums of hydrogen peroxide to mix explosives. Police surreptitiously replaced it with a diluted chemical to thwart the men.

Chemists said the trio could have mixed 550 kilograms of an explosive with the same blast effect as 400 kilograms of TNT.

Federal magistrates sitting on the case have said the trio had planned to detonate several bombs "in the near future."