BERLIN (AP) — Two Iranian men have been detained in Germany following a tip from U.S. security officials that at least one of them could be planning an attack with deadly chemicals, officials said Sunday.

Police and prosecutors said the brothers, aged 32 and 25, were detained overnight in the town of Castrop-Rauxel, northwest of Dortmund.

The authorities said in a joint statement the men are suspected to have planned a serious attack motivated by Islamic extremism, for which they had allegedly sought to obtain the potent toxins cyanide and ricin.

Specialists wearing anti-contamination suits were seen carrying evidence out of the older man’s home. Duesseldorf prosecutors later said an initial search of the premises turned up no toxic substances.

It wasn’t immediately clear how far advanced the plans for an attack were and whether the suspects had picked a specific target, but prosecutors said they would ask a court to keep the men jailed pending further investigation.

If convicted of jointly arranging a deadly attack, the men — who were identified by prosecutors only as M.J. and J.J. due to German privacy rules — could face between three and 15 years imprisonment.

German news agency dpa quoted the top security official for North Rhine-Westphalia state, Herbert Reul, as saying authorities had received “a serious tip that led police to intervene the same night.” Duesseldorf prosecutors said the information came from authorities in the United States, but declined to elaborate.

Germany’s top security official thanked police and specialists from the country’s disease control agency who had taken part in the raid.

“Our security services take any information about Islamist terror threats very seriously, and act,” Interior Minister Nancy Faser said in a statement, adding that 21 Islamist attacks had been prevented in Germany since the start of the century.

Faeser noted the importance of international cooperation in fighting extremist threats and said further investigations by Duesseldorf prosecutors would show whether the suspicions that triggered the police operation were warranted.

Five years ago, German police arrested a Tunisian man and his wife on suspicion of planning to carry out a ricin attack in the name of the Islamic State group. They were later found guilty and sentenced to 10 and eight years’ imprisonment, respectively.

Even small amounts of ricin, which is produced from the seeds of castor oil plants, can kill an adult if eaten, injected or inhaled.