A woman walks past a JP Morgan Chase bank branch in New York, October 15, 2008. REUTERS/Lucas Jackson

WASHINGTON (Reuters) - At least 30 Chase bank offices in nine U.S. states have been targeted by threatening letters, many of them containing an unidentified powder, an FBI spokesman said on Tuesday.

At least five Chase branches in Colorado and Oklahoma received the letters on Monday and some contained a white powder, Chase had said.

“Today we’re well over two dozen,” FBI spokesman Rich Kolko said. “To the best of my knowledge they’re all Chase.” An FBI statement later said more than 30 letters were being investigated.

He declined to characterize the threats but said he was unaware of any connection to recent troubles in the U.S. financial system.

“At this point, field tests on the powder have been negative,” for harmful substances, Kolko said. He said local laboratories will do more testing.

Chase spokesman Tom Kelly said there have been no injuries to bank employees or customers, and referred additional questions about the probe to the FBI.

The FBI, U.S. Postal Inspectors and local authorities are investigating the letters, he said. The letters have been received at branches in New York, New Jersey, Washington, D.C., Illinois, Ohio, Oklahoma, Colorado, Georgia and Texas.

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

JPMorgan Chase & Co last week surpassed Citigroup Inc to became the largest U.S. bank. It has aggressively acquired other assets as the financial system has weakened, including the banking assets of Washington Mutual Inc.