A spokesman for Berlin prosecutors confirmed they had launched an investigation over suspected incitement to racial hatred.
The letters, signed by an "officer for the deportation of foreigners," were presented as a "notice" to candidates of Turkish origin ahead of elections on September 27.
According to one recipient, Green politician Ozcan Mutlu, the two-paged letter contained a "five-point plan" for "moving foreigners gradually back to their home countries."
The letters sparked outrage from mainstream politicians who called for renewed efforts to outlaw the NPD, which has no seats in the national parliament but is represented in two of Germany's powerful regional assemblies.
The NPD is on the verge of bankruptcy and has next to no chance of winning a seat in Sunday's election, polls show.