An envelope containing poison was mailed to the Czech Republic's interior minister.
After conducting tests Radek Pokornik, a spokesman for the National Institute for Nuclear, Chemical and Biological Protection said the envelope contained a "a cyanide-like material in a life-threatening amount".
Pokornik did not give any further details.
The minister, Milan Chovanec, said the envelope was sent to him at the ministry from a Nordic country he did not identify.
It was intercepted by security before it could be opened.
The country's foreign ministry, the government, and the presidential office all recently received suspicious envelopes which contained a white powder that was not found to be poisonous.
Chovanec said he did not believe that the two incidents were connected.
He said that no staff had been injured, and that there was no need to review security procedures.
He told Czech media that the substances used in the incidents were different and the envelope sent to him contained a letter, but did not disclose its contents.
He said that in the Interior Ministry they "make many important decisions and tread on the heels of criminals. For instance, we started to fight the mafia and supported our Kurdish allies in the fight against Islamic State".
President Milos Zeman said that the letter addressed to him was written in the name of Islamic State, and protested the Czech government supplying arms to the Kurds.