updated 10/22/2008 6:24:19 PM ET 2008-10-22T22:24:19

At least a dozen more suspicious letters turned up Wednesday at financial institutions nationwide, but all appear to be harmless, the FBI said.

The U.S. Postal Service offered a reward of up to $100,000 for help in arresting those who mailed a total of nearly 50 threatening letters over the last several days to Chase Bank branches and federal regulatory offices in at least 11 cities.

Some of the letters were filled with white powder that has so far tested negative for poisonous or otherwise dangerous toxins, FBI spokesman Richard Kolko said in Washington.

An FBI spokesman in Oklahoma, where eight letters turned up, said local preliminary assessments showed the powder was harmless calcium powder.

The letters were mailed to banks and financial institutions in and around Atlanta, Chicago, Columbus, Ohio, Dallas, Denver, Newark, N.J., New York City, Oklahoma City, Phoenix, San Francisco and Washington.

The letters first began surfacing Monday, forcing some bank branches to close. No injuries were reported, but some Chase employees, including a pregnant woman, were examined as a precaution. Chase branches around the country "are on alert," said JPMorgan Chase spokesman Greg Hassell.

Authorities said the letters appear to be from the same source, and were focusing on possible suspects near Amarillo, Texas, where the envelopes were postmarked.

Copyright 2008 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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