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

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

Authorities said in a joint statement that they were suspected of planning a serious attack inspired by Islamic extremism, for which they sought to obtain the powerful poisons cyanide and ricin.

Specialists in anti-pollution suits were seen taking away evidence from the old man’s home. Dusseldorf prosecutors later said that an initial test of the compound had found no toxic substances.

It was not immediately clear how far advanced the plans for the attack were and whether the suspects had chosen a specific target, but prosecutors said they would ask the court to jail the men pending further investigation.

The men – identified only as MJ and JJ by prosecutors due to German privacy rules – could face between three and 15 years in prison if convicted of jointly organizing a deadly attack.

German news agency dpa quoted Herbert Riel, the top security official in the state of North Rhine-Westphalia, as saying authorities received “a serious tip that led to police intervention that same night.” Prosecutors in Düsseldorf said the information came from authorities in the United States, but declined to elaborate.

Germany’s top security official thanked police and experts from the country’s disease control agency for taking part in the test.

“Our security services take any information about Islamic terrorist threats very seriously and act on them,” Interior Minister Nancy Fasser said in a statement, adding that 21 Islamist attacks have been prevented in Germany since the turn of the century.

Fesser noted the importance of international cooperation in combating terrorist threats, and said further investigations by Düsseldorf prosecutors would show whether the suspicions that prompted the police action were justified.

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

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