Federal authorities announced that a North Texas man believed to be responsible for sending some 500 threatening letters containing a white powder to various public entities is now in custody, The Associated Press reports. While the letters and the powder contained within proved to be non-toxic hoaxes, that didn’t stop countless dollars and hours from being spent on their disposal and investigation. Hong Minh Truong, 66, made his initial appearance in Dallas federal court Monday.

The letters included vague, puzzling messages, like the one sent in May 2012 that read “Al Qaeda back! Special thing for you… Where are you Scooby Doo, Counter Intelligence, CIA, you do not know how to catch the triple dealer spy in your law enforcement.”

It’s alleged that Truong began sending the letters in batches beginning in 2008, with an eventual total of 15 batches. All but two of the batches from the Dallas area contained white powder. Truong will be held in custody until a detention meeting on August 4.

“While it was determined that the mailings did not contain toxins or poisons, each incident required a field screening of the letter’s contents, which cost taxpayer dollars and diverted first responder resources,” Diego Rodriguez, special agent in charge of the FBI’s Dallas office, said in a statement.

Truong wasn’t identified until 2008, when investigators were able to link him to an IP address following threatening letters sent to 20 Boston-area schools.