Dozens of hedge funds are reported to have received subpoenas from the Securities and Exchange Commission as it investigates rumours that have battered the share price of Lehman Brothers in recent weeks.

The SEC, which is focusing on four specific rumours that may damaged the investment bank's credibility in the market, according to the Wall Street Journal, has asked the funds for transcripts of phone calls, messages and payroll documents that mention the Federal Reserve's lending facility, Barclays, hedge fund SAC Capital Advisors and fixed-income manager Pimco.

The investigation into potential market manipulation involving rumours about Lehman's supposedly deteriorating financial health followed the near-collapse of another investment bank, Bear Stearns, which was the target of similar rumours and was sold in extremis to JPMorgan Chase when it appeared that it would be unable to find financial counterparties with which to trade.

Europe has seen similar cases. The UK's Financial Services Authority recently launched an unprecedented investigation into dealings in the shares of major financial institutions trying to raise capital from shareholders amid suspicion that speculators have been spreading false rumours to force down the value of shares in mortgage bank HBOS.

However, hedge funds and other traders retort that some of the rumours have turned out to be true. For example, the subscription rate for HBOS's capital-raising turned out to be meagre, with less than 10 per cent of shareholders backing the rights issue.

Rumours have always played a part in financial markets, and the authorities on either side of the Atlantic have yet to find hard evidence that they are being deliberately used to manipulate markets or ruin financial institutions.