Updated: 2:01 p.m. 5/17

DALLAS (CBSDFW.COM/AP) — Two more envelopes containing white powder turned up in North Texas Thursday morning.

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Dallas police say the one found at the Crowley Courts building was sent from a prisoner to a judge there.  They say it’s unrelated to hundreds of others sent across the country.

That envelope was found at 8:33 a.m. and three people were “sheltered in place” because they were near the envelope when it was opened. Nobody was hurt, Dallas Fire Rescue spokesman Jason Evans said in a release.

An envelope found Thursday at 8:45 a.m. at a packing and shipping business in Plano also caused authorities to block the entrance to the shopping center where the business is located, at 18208 Preston Road.

At the scene, the U.S. Postal Inspector said there was no white powder found in the letter –– contrary to what was originally reported. It is not related to the large-scale FBI investigation, the inspector said. Authorities would not say why the letter spurred the blockade.

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The FBI says it suspects the mailings of the envelopes 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-Qaida 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 daycare 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.

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Copyright 2012 CBS Local.  All rights reserved.  The Associated Press contributed to this report.