Austrian authorities say two teenage Syrian brothers have been arrested at a refugee center in the country on suspicion of involvement with terrorist organizations — in one case, the Islamic State group.

Prosecutors in Graz said Wednesday that the two, aged 16 and 18, were arrested Dec. 17 in the nearby town of Lebring and remain in custody, the Austria Press Agency reported. They said that the tip leading to the arrest came from Germany, where authorities accuse a third, elder brother of belonging to IS.

Of the two arrested in Austria, one is suspected of having fought for IS in Syria and the other of belonging to Ahrar al-Sham, an ultraconservative group that operates mainly in northern Syria. The brothers weren't identified.