Two Czech ministers from the centre-left government received envelopes containing poison last week, a local official and media said on Monday.
Interior Minister Milan Chovanec of the ruling leftwing Social Democrats and Finance Minister Andrej Babis, the billionaire head of the centrist ANO party, both received toxins in the post.
"The letter sent to the finance ministry contained dangerous poison in a lethal amount," Radek Pokornik, spokesman for the National Institute for Nuclear, Chemical and Biological Protection, told AFP.
He said he could not comment further because of the ongoing investigation.
The Czech news agency CTK quoted Babis as saying the envelope arrived at the ministry last Thursday and had been sent from Slovenia.
Chovanec meanwhile received an amount of cyanide that could be life-threatening last Tuesday in a letter sent from northern Europe, CTK said.
The president, government and foreign minister had already received envelopes containing non-dangerous white powder in September.
The three-party coalition government, which also includes the centrist Christian Democrats, has been in power since the early elections of October 2013.