Washington, D.C. ? 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 about 50 threatening letters over the last several days to Chase Bank branches and federal regulatory offices in 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.