DALLAS â€"  The FBI says it suspects the mailings of hundreds of envelopes containing white powder over the last four years are the work of someone in Texas.

The FBI released a statement Wednesday saying the letters contain postmarks from North Texas and similar references to subjects such as Al Qaeda and Nazis. A $150,000 reward is being offered for information leading to a conviction in the case.

More than 380 threatening letters in envelopes containing white powder have shown up across the U.S. and overseas since 2008. Last week, more than 20 were mailed to day care centers, elementary schools and an aerospace-related business in Texas and other states.

Tests showed the powder wasn't hazardous.

The letters also have been sent to government offices, churches, schools, restaurants and U.S. embassies overseas.