Nearly a year after the flash crash on May 6, 2010, there are still no guarantees it won’t happen again.

Yesterday, Securities and Exchange Commission chief Mary Schapiro said that while steps have been taken to prevent wild swings — such as eventually introducing market-wide circuit breakers — she couldn’t rule out a repeat crash.

In a matter of minutes last May, amazed and frightened investors watched as markets lost $1 trillion in value.

“Can I guarantee we will never have another flash crash? No,” Schapiro told reporters during a Senate appropriations subcommittee hearing.

Her remarks came two days before the anniversary of one of the most gut-wrenching trading days in US history, which was traced to a single trade made by a mutual fund in Kansas.

That led to a dramatic 20-minute roller-coaster ride, sending stocks of well-known healthy corporations trading for pennies before whipsawing back to post-crash levels.

“A lot has been done, but there’s still a lot that needs to be addressed,” said Joe Mecane, executive vice president at the New York Stock Exchange.

Market-wide circuit breakers would limit by 10 percent the range that a stock could move before being halted in trading.

One of the biggest criticisms of the flash crash was that the various exchanges, including the NYSE and the Nasdaq, employed different circuit-breaker mechanisms, which increased the severity of the tumult.

An improvement, known as “limit up and limit down orders,” is under review and could be implemented by the fourth quarter. Those rules would prevent some orders from even taking place.

The planned changes come during the high-stakes fight for control of the NYSE, which has agreed to a merger with Germany’s Deutsche Boerse, while the Nasdaq and IntercontinentalExchange are planning to take their bid directly to NYSE shareholders.