VIENNA (Reuters) – Austrian police took eight people into custody on Thursday in raids linked to potential connections with the militant group Islamic State, prosecutors in the city of Graz said.

Around 800 police officers took part in the raids in Vienna and Graz “due to suspected participation in a terrorist organization (‘IS’),” they said in a statement, adding that the coordinated action had been planned for some time.

The statement gave no more details, but a spokesman said the people taken into custody included three Austrian nationals with a migrant background, two Bosnians and a Syrian. The nationalities of the other suspects were not immediately known.

“There was no acute danger” and no indications of a concrete attack, the spokesman said, adding that the detentions were not connected to the arrest of an Austrian teenager last week on suspicion of planning an Islamist attack in Vienna.

That suspect, a 17-year-old with Albanian roots, was arrested on Friday after tip-offs from unspecified foreign countries. Austria alerted Germany to a related suspect, a 21-year-old who was arrested in the western German city of Neuss on Saturday. A boy thought to be 12 has also been held in Austria.

German authorities have been on high alert since a Tunisian failed asylum seeker rammed a truck into a Christmas market in Berlin on Dec. 19, killing 12 people.

Police in Vienna have also been on heightened alert since Friday’s arrest and have increased patrols at transport hubs and busy public places.

(Reporting by Alexandra Schwarz-Goerlich in Vienna and Michael Shields in Zurich; Editing by Gareth Jones)