Jamie Dimon, the chief executive of JPMorgan Chase, and dozens of his executives have received death threats, all thought to be from the same person, accusing the bank of stealing Washington Mutual.

Mr Dimon and JPMorgan executives at branches in 11 US cities, and banking regulators have received at least 50 anonymous letters, which express outrage about WaMu’s recent sale to JPMorgan.

As it teetered on the brink of bankruptcy last month, WaMu was forced into a firesale by the US Government, for a fraction of the price that JPMorgan had offered for the group in March, when its advances were turned down.

Many of the letters contained a white powder, which turned out to be harmless calcium, and the warning: “Steal tens of thousands of people's money and not expect reprercussions [sic]," read one, which was written in all capital letters and includes the misspelling.

"It's payback time. What you just breathed in will kill you within 10 days,” the letter continued.

Mr Dimon is thought to have received a variation on that letter, with a different typo: “You need to be pay back. You will be killed in ten days,” it said.

The writer also threatened a series of attacks on targets connected to Mr Dimon and JPMorgan, including a bombing described as being similar to that in Oklahoma City, where 168 people were killed in 1995.

It is understood that the letters, bearing an Amarillo, Texas postmark, were sent out on October 17 and 18. They were received by Chase bank branches in Atlanta, Chicago, Columbus, Ohio, Dallas, Denver, Newark, N.J., New York City, Oklahoma City, Phoenix, San Francisco and Arlington, Virginia.

They were also sent to regulators such as the Federal Deposit Insurance Corporation and the Office of Thrift Supervision.

The US Postal Inspection Service has offered a $100,000 (£61,400) reward for information relating to the letters. An FBI spokesman said, "We are following several good leads, but we are always looking for more information.

“And we hope that when people see this letter and writing style, it will encourage someone with information to contact the FBI or other authorities," he added.

JPMorgan bought the bulk of WaMu from US regulators last month for $1.9 billion in a deal that will wipe out the equity holding of shareholders and leave many bondholders out of pocket.

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