By Jim Forsyth

SAN ANTONIO, Texas (Reuters) - 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.

No one was injured in the incidents last week where envelopes containing a similar nonhazardous substance were mailed to elementary schools, early childhood development centers and an aerospace-related business.

However, the incidents disrupted school and production days and forced authorities to scramble emergency crews trained in handling hazardous materials.

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 is offering a reward of up to $150,000 for the arrest of the suspect.

The mailings began in late 2008, and have been directed to schools, government offices, churches, day care centers, U.S. embassies abroad, restaurants and other private businesses. Each envelope has held a similar nonhazardous substance.

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."

The letters enclosed "continue to reference subjects such as al Qaeda and the Nazi SS, which is believed to be intended for shock value rather than to express any sincere sympathy or affiliation," Kolbye said.

One letter released by the FBI included the phrasing "Al Qaeda back! Special thing for you! What the hell, where are you, Scooby Doo, Counter intelligence, CIA, you do not know how to catch the triple dealer spy in your law enforcement ..."

Kolbye said the suspect may use "unusual phrases" like those in daily conversation.

The FBI has advised the public to take precautions if they receive such envelopes and treat the white powder with caution even though no hazardous materials have been found in the mailings so far.

(Reporting by Jim Forsyth; Editing by Lisa Shumaker)