Andrej Holm, a sociologist at Berlin's Humboldt University, has been held in solitary confinement since his arrest July 31, according to the Guardian newspaper.

Holm and another academic, identified only as Mathias B., and two other Berliners all had their homes and workplaces searched following the arrest of three militants charged with attempted arson of German military vehicles.

According to police documents, all four of the Berliners had been under surveillance since September 2006, the Chronicle of Higher Education reports.

Holm is accused of membership in a terrorist organization under a controversial provision of German law dating back to the 1970s that gives prosecutors the right to charge people “suspected of having given intellectual or ideological guidance to people who have committed a crime, if the prosecutors can demonstrate some sort of intellectual or ideological link between them," one of Holm’s supporters, Neil Brenner, a professor of sociology at New York University, told the Chronicle.

The arrest warrant for Holm says that he met with the three accused arsonists in “conspiratorial circumstances” -- leaving his mobile phone at home.

The charges against Matthias B., a political scientist at the Free University of Berlin, allege that he has used "phrases and key words" in his academic writings that are also used by the alleged militants.

Dozens of academics from all over the world have signed two open letters to Germany's federal prosecutor, protesting the charges.