The Federal Bureau of Investigation is hoping that a reward will draw out clues on who is behind a series of bomb hoaxes outside military recruiting centers.

The $20,000 reward was announced Wednesday.

FBI officials said fake pipe bombs were planted outside recruiting centers in several Oregon cities, including Portland.

Each of the devices had a German phrase on it, which translated to “the white rose.” Investigators said that phrase refers to a small group of anti-Hitler activists in World War II.

The threats date back to March 2007.