An investigation into last week's mysterious plunge in US share prices has found no single cause was to blame.
The Securities and Exchange Commission (SEC) said it still needed more time to investigate, but warned that new safeguards would be introduced.
SEC chairman Mary Schapiro told a Congressional hearing that the markets had "failed" many investors.
On 6 May, the Dow Jones fell a lightning-fast 700 points, sending shivers through the investor community.
Theories about the possible cause ranged from a rogue trader to terrorism. All have been discounted, the SEC said.
Mrs Schapiro told the hearing that it would take time to pinpoint the cause and warned that investor confidence could suffer if there were no reforms.
"The markets failed many investors on May 6, and I am committed to finding effective solutions in the very near term," she said.
Although she suggested that the cause was possibly due to a confluence of events, no regulator or exchange has provided a full account of events or concluded what caused the drop.
"The sudden evaporation of meaningful prices for many major exchange-listed stocks in the middle of a trading day is unacceptable and clearly contrary to the vital policy objective of maintaining fair and orderly financial markets," Mrs Schapiro said.
It seems likely that the SEC will now focus its investigation on computer-driven high-frequency trading (HFT), in which share dealing is carried out a rapid speed.
More than 100 SEC staff are probing last week's events, and sifting through more than 17 million share trades.
Reports has suggested the SEC was looking specifically into trades in Procter & Gamble, but Mrs Shapiro said nothing unusual was found.
On Monday, the heads of major exchanges and the SEC agreed to strengthen existing circuit breakers that are designed to stop HFT exacerbating share price falls.
It is possible that the additional safeguards will include circuit breakers that would trigger a pause in trading if a share price fell more than 10% in a five minute period.