VIENNA (Reuters) – An Austrian teenager arrested on suspicion of planning an Islamist attack in Vienna has told investigators he built a “test bomb” in Germany, where another suspect has been arrested, Austria’s interior minister was quoted as saying on Tuesday.

The Austrian 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 city of Neuss on Saturday. A boy thought to be 12 has also been held in Austria.

Whether the German and Austrian suspects are believed to have planned separate attacks or a joint one, and of what nature, is not clear. Austria has said public places in Vienna including its underground transit system might have been a target.

“A test bomb seems to have been put together,” Austrian Interior Minister Wolfgang Sobotka told broadcaster ORF, even though no explosives were found in the apartment in question. “That is all we can announce today from the questioning.”

Asked what he meant by a test bomb, Sobotka said: “Where one tries to put together materials obtained on the market from instructions on the internet.” He added that what had been established in the questioning was changing daily.

An Interior Ministry spokesman declined to elaborate.

The German admitted during questioning that the Austrian had visited him for two weeks at the end of last year, a spokesman for the Duesseldorf prosecutor said on Monday.

Germany’s Focus magazine had said the man was planning a bomb attack on police and soldiers. Both he and the Austrian had experimented with materials to create explosives in the Neuss apartment, it said.

German authorities have been on high alert since a Tunisian whose bid for asylum had been rejected rammed a truck into a Christmas market in Berlin on Dec. 19, killing 12 people.

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

(Reporting by Francois Murphy and Alexandra Schwarz-Goerlich; Editing by Janet Lawrence)