First analysis of the substance suggested it was harmless soda powder, police said Thursday.
The text contained spelling errors, raising doubt whether it was sent by Islamists or far-right activists trying to stoke fear, said one of the recipients, Greens party lawmaker Hans-Christian Stroebele.
Berlin police told AFP that five letters -- one addressed to Merkel's husband Joachim Sauer -- were received Wednesday and Thursday.
The others were sent to the head of Merkel's CSU Bavarian sister party, Horst Seehofer, the far-left Die Linke MP Gregor Gysi, and the Greens' top candidate Katrin Goering-Eckardt, said public broadcaster NDR and WDR.
A government spokesman said that "we do not comment of security matters". Police said that the domestic security agency BfV was investigating.
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