BERLIN (AP) — Police in western Germany raided the apartments of alleged Islamic extremists on Thursday and detained six people amid suspicions the group may have been planning an attack.

Cologne police said the raids, which started at around 4 a.m., centered on an apartment in nearby Dueren where the main suspect in the case had recently moved, joining another radical who already lived there, and on his construction company in Cologne. An apartment where he previously lived in Berlin also was searched.

The main suspect is a 30-year-old German-Lebanese convert to Islam who has been on authorities' radar as an extremist for six years and tried repeatedly to travel to territory controlled by the Islamic State group, senior police official Klaus-Stephan Becker told reporters.

Police acted after the man was heard talking about "planning to ascend to the highest level of paradise of the Muslim faith,"

That "could be a synonym for a suicide attack," Becker said.

Police felt that they had to move in as a precaution but so far have no concrete indications of where, when and how a possible attack was to be carried out, he said.

Cologne police chief Uwe Jacob said he was "confident that we will soon be able to substantiate our threat assessment."