U.S. federal authorities are investigating at least 30 threatening letters containing white powder sent to Chase banks in eight U.S. cities though all the packages that have been tested appear to be harmless.

The Federal Bureau of Investigation said Tuesday initial tests on the powder have come up negative for dangerous toxins. Additional tests are ongoing.

"These threat letters contain a powder substance," the FBI said in a statement. "At this point, field tests on the powder have been negative. Additional testing will be completed. Even sending a hoax letter is a serious crime."

A law enforcement official said Tuesday the letters were mailed to Chase bank branches in Chicago, Cincinnati, Dallas, Denver, Newark, New Jersey, Oklahoma City and Washington D.C.

The letters all appear to be from the same source and began showing up at the banks on Monday, the official said. They were sent from South Texas, said the official, who spoke on condition of anonymity because he was not authorized to discuss the matter publicly.

The U.S. Postal Service and state and local officials are also investigating. Postal inspector JoJan Henderson says the letters appear to be related.

No injuries were reported but some employees, including a pregnant woman, were examined as a precaution, said JP Morgan Chase & Co. spokeswoman Mary Jane Rogers.

Neither the local Postal Service nor the FBI has released the text of the letters or their origin.

Letters sent to several banks in Denver, Oklahoma City, New Jersey

Eight banks in the Denver area and eight in the Oklahoma City area received letters containing white powder, officials there said. All Denver branches reopened Tuesday.

In Oklahoma, the state Department of Health is conducting tests on the substance found in the notes, but Gary Johnson, an FBI spokesman there, said preliminary assessments done locally determined it to be harmless calcium.

Johnson said the Oklahoma letters indicated that the threat was "based on past actions of the bank" and that the letters implied that the opener was going to die.

Nine branches in New Jersey and a credit card centre in Elgin, Ill., about 50 km northwest of Chicago, also received similar threatening letters, said Greg Hassell, a JP Morgan Chase spokesman in Houston.

Hassell said authorities were in the process of clearing those locations, and some of the nine New Jersey branches were still closed as of Tuesday afternoon.

All the suspicious mail that has been tested has turned out to be harmless, Hassell said, but other Chase branches around the country "are on alert."