BERLIN — Federal prosecutors in Germany said Wednesday that they have brought terrorism charges against two Lebanese brothers and a German man accused of being involved in a radical Islamic group fighting in Syria.

The charges come amid growing concern across Europe about the threat posed by local jihadists returning from the front. The police detained the main suspect, a 24-year-old Lebanese man identified only as Ismail I., at a highway rest stop in southwestern Germany last year, prosecutors said.

Officials said he had returned to Germany from Syria, where in August 2013 he is suspected of joining and training with the Islamic State of Iraq and Syria, or ISIS, a Qaeda affiliate.

German authorities believe some 300 people have left Germany for Syria since the conflict began there, and the authorities worry that the dozen people who they say have returned radicalized pose a threat of increased terrorism. Officials have said those who have returned are capable of building bombs and carrying out attacks, although they have no indications of concrete plans.