Washington, D.C. ? More than 30 letters containing a suspicious powder were mailed to Chase bank branches and federal banking regulators’ offices in nine cities, authorities said Tuesday in what was being investigated as a first, if extreme, public backlash over the nation’s financial crisis.

Initial tests on the powder proved negative for poisonous or otherwise dangerous toxins, the FBI 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 Tuesday as officials zeroed in on possible suspects near Amarillo, Texas, where the letters were postmarked.

“Most of these letters contain a powder substance with a threatening communication,” the FBI said in a statement.

“Even sending a hoax letter is a serious crime,” the FBI said.

A law enforcement official said the letters were mailed to Chase bank branches in or near Atlanta, Chicago, Columbus, Ohio, Dallas, Denver, Newark, N.J., New York City, Oklahoma City and Washington. They all appear to be from the same source and began showing up at the banks on Monday.