An official said an envelope mailed to the Czech Republic's finance minister contained poison, the second such case in a week.
Tests conducted on the suspicious envelope to minister Andrej Babis detected "a deadly amount of a dangerous poison," according to Radek Pokornik, a spokesman for the National Institute for Nuclear, Chemical and Biological Protection, AP reported.
Pokornik declined on Monday to give further details about the letter received Thursday, citing a police request.
Tests conducted by the institute found that an envelope mailed to Interior Minister Milan Chovanec on Tuesday contained a life-threatening amount of cyanide-like material.
Both envelopes were intercepted before anyone could be exposed.
Pokornik said another suspicious envelope mailed to the presidential office last week did not contain poison.