It's always unsettling when envelopes containing white powder start showing up in mailboxes, because even though it's probably baking soda, it could always be good ol' anthrax.

The 20-plus powder-filled letters that showed up at schools, childhood development centers and aerospace-related businesses last week were apparently harmless, but the FBI is not laughing. Even if one of the letters referenced Scooby Doo.

The FBI finds last week's scare even less funny because agents believe the same person has mailed more than 380 such letters since 2008 to schools, churches, government offices, embassies, all with a North Texas postmark. So unfunny in fact that it's ponying up $150,000 for information leading to the sender's arrest.

From what law enforcement has pieced together, the sender is probably a male, probably over 30, and considered odd or eccentric by others. He may have a history of mental health problems and be "unusually fascinated" with conspiracy theories.

Which, come to think of it, does ring a bell.