More than 30 letters containing a suspicious powder were mailed to Chase banks in nine US cities, authorities said Tuesday in what was being investigated as a first, if extreme, public backlash over the financial crisis in the United States.

Initial tests on the powder proved negative for poisonous or otherwise dangerous toxins, the Federal Bureau of Investigation said. An FBI spokesman in Oklahoma, where eight letters turned up, said local preliminary assessments showed the powder was harmless calcium.

Additional tests were being run on the letters yesterday as officials zeroed in on possible suspects near Amarillo, Texas, where the letters were postmarked.