Matthias Heyder, who hopes to lead the far-right party into the state legislature on March 20, is under suspicion of using an online forum to discuss possible terrorist attacks, bomb-making methods and the "defiling" of left-leaning women under the pseudonym "Junker Jörg."
The story, first broken by public broadcaster ARD, was confirmed by the Saxony-Anhalt Interior Ministry on Tuesday.
"According to the excerpts of this internet forum made available to us, it's very plausible that no one other than Mr. Heyder is behind these entries," said Martin Krems, a spokesman for the ministry.
The state police have now been asked to secure computer data and other information.
The forum itself is registered to Holger Apfel, the NPD parliamentary group leader in Saxony. Apfel, who is also heading the party's election campaign in Saxony-Anhalt, is a close confidante of Heyder.
Investigators have found hundreds of entries made by "Junker Jörg" since 2004 on the forum, including a good deal of insider information about the NPD such as detailed postings on campaign strategy, photographs to be used in campaign material and drafts of party brochures.
Entries which concern investigators were found under the subcategory "weapons" and explain in seven steps how an explosive device could be assembled. Experts believe the detailed description, which includes exactly how much of various substances should be used, is punishable by law.
In one entry, Junker Jörg wrote: "20 suitcases, 20 men, 20 train stations. The Federal Republic paralysed. Everything legal. Cost under €1,000. What's the problem?"
Heyder, when contacted by ARD, said he doubted he had ever written anything in the forum in question. He did tell reporters, however, that the forum was private and threatened to press charges for "data theft" if postings were made public.
According to recent polls, the NPD could get enough votes in Sunday's election to gain seats in the Saxony-Anhalt state parliament. It would be the third eastern German state to have members of the neo-Nazi party represented after Saxony and Mecklenburg-Western Pomerania.
Heyder and the party have been careful to cultivate a moderate image in the election campaign, however, discussions in the internet forum where Junker Jörg often posted smirked at the efforts while concluding that they were necessary politically.