Investigators say 25 white powder hoax letters mentioning al-Qaida have been received by churches, mosques and businesses in the Dallas area since Aug. 5.

The FBI and the U.S. Postal Inspection Service on Friday announced a reward of up to $100,000 for information leading to the arrest and prosecution of those responsible.

FBI Special Agent Mark White says the north Texas-postmarked letters are in English, typewritten with just one line. He says the message is unclear, but the letters are considered threatening. The powder tested negative for toxins.

White says five additional letters have been received, in Austin, Lubbock, Chicago and Waltham, Mass.

He says all of the notes are similar to threatening letters sent to U.S. Embassies in Europe and some governors since December 2008.