Authorities believe one person is responsible for mailing 20 powder-filled envelopes to schools and a business in the Dallas-Fort Worth area last week and hundreds of similar letters around the world since late 2008, the FBI said on Wednesday…

All of the letters have been sent through the U.S. mail and all were postmarked from north Texas, said Kevin Kolbye, acting special agent in charge of the Dallas FBI office.

“The sender has taken steps to conceal his activities and prevent law enforcement from finding physical evidence of his crimes, including his fingerprints,” Kolbye said…

The FBI said the mailer likely is male, “may not demonstrate a mastery of formal English, is likely over the age of 30, and may be unusually fascinated with certain topics such as the FBI, intelligence, terrorism, conspiracy theories and aerospace.”