There is no telling at this point whether these were sent by genuine mujahedin or just some nut — not that it much matters for their victims if they follow through with action. From the National Terror Alert Response Center (thanks to Morgaan Sinclair):
Investigators in Marion County, Fla., are searching for the author of nine postcards sent to different schools on the same day, with the words “˜Jihad-Boom” and cartoons drawn by hand, of a building being blown up with people inside.
Investigators released the postcards with the threatening drawings on them Monday in hopes of generating leads in the case. Officials said that the postcards are made up of various and traditional themes, and each one has a distinctive hand-drawn cartoon on it. Several of the threats arrived on postcards featuring Walt Disney World. Detectives said the threat-maker crossed his or her No. 7s and attached a suffix to the address. Six of the nine postcards spell the word “Jihad” correctly, while the others are incorrect.
The U.S. Postal Service said that the postcards were all mailed to the schools from the Ocala-Gainesville mailing district before the sixth anniversary of the Sept. 11 terrorist attacks. A $10,000 reward has been issued to anyone who provides information which leads to the arrest and conviction of the person or persons responsible for using the postal service to mail the threatening postcards to schools.