German police conducted raids and arrested two suspects on Thursday in an investigation of four Algerian men who are suspected of planning attacks in Germany and having ties to the Islamic State group.

The arrests were made in Berlin and at a refugee home in the western town of Attendorn, Berlin police spokesman Stefan Redlich said. The arrests were based on existing warrants in other cases, and a woman who isn't under investigation in the current case also was detained on an unrelated warrant, he said.

The other two suspects, one of whom was at a refugee home in the central city of Hannover, weren't arrested.

Authorities suspect that the four men had contacts with the IS group and say that one of them — the man arrested in Attendorn — was being sought by Algerian authorities for belonging to the extremist group. He is believed to have received military training in Syria.

Redlich said the probe started in December and there was no specific trigger for staging the raids on Thursday, noting that searches in multiple locations require preparation. Investigators seized computers, cellphones and other material.

"We hope for further information from the evaluation of the evidence as to whether this plan existed, how far along it was and how concrete, with what target," Redlich said.

He wouldn't comment on where word of the possible attacks came from. It also wasn't immediately clear how long the suspects had been in Germany.

The two men arrested on Thursday have used aliases, claiming to be Syrian and French, Redlich said.