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.
The typed letters warned that the content was "deadly", mentioned the ultra-orthodox Muslim Salafist community and were signed "Allahu Akbar" (God is Greatest), said the report.
A government spokesman said that "we do not comment of security matters". Police said that the domestic security agency BfV was investigating.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)