German prosecutors said Tuesday they are investigating two men suspected of planning terrorist attacks using model airplanes, and authorities in Germany and neighboring Belgium conducted a series of searches.

No one was arrested in Tuesday's raids, which were carried out to gather "evidence for possible attack plans and preparations" and information on terrorist financing, federal prosecutors said in a statement.

Prosecutors said the investigation involves possible charges of "preparation of a serious, state-threatening act of violence," but not membership of or supporting any terrorist organization.

In all, nine properties were searched in the Stuttgart and Munich areas of southern Germany, in eastern Germany's Saxony state and in Belgium.

The two men of Tunisian origin are suspected of "procuring information and objects to commit Islamic extremist explosive attacks with remote-controlled model airplanes," prosecutors added. They gave no further information on the two men and didn't identify them.

The apartments of four acquaintances of the men, suspected of financing Islamic extremism, were searched in Germany. The investigation also targets a further acquaintance suspected of money laundering. None of the suspects was identified.

Last November a U.S. man, Rezwan Ferdaus, was sentenced to 17 years in prison over a plot to fly remote-controlled model planes packed with explosives into the Pentagon and U.S. Capitol.

Germany has seen only one successful attack by an Islamic radical -- the fatal shooting of two U.S. airmen at Frankfurt airport in 2011 by a Kosovo native who grew up in Germany and became radicalized on his own by watching jihadist propaganda on the Internet.

However, there have been several attempted attacks in the country, which is a major contributor to international forces in Afghanistan.

Separately Tuesday, French authorities said police detained six people in the Paris region on suspicion of plotting terrorist attacks in France.