When Dick Grasso, the then-chairman of the New York Stock Exchange, reopened trading on September 17th, 2001, it was a symbol of Wall Street's resilience in the face of terrorism.

But the four-day closure of the exchange also underlined the serious disruption caused by the attacks on the World Trade Center, due partly to flaws in Wall Street's disaster planning.

Five years and billions of dollars of investment later, many of those flaws have been addressed and there is little doubt that financial markets in New York - and in other leading financial centres - would now be more resilient in the event of a similar disaster.

But concerns remain, not least about the key utilities on which the financial services sector depends and the industry's ability to handle other sorts of threat, such as a flu pandemic.

Scott Parsons, deputy assistant treasury secretary for critical infrastructure, says the most basic lesson learned from September 11th was the need for back-up facilities far enough away from a main site.

Most companies' contingency plans had focused on localised problems. "Some firms had their back-up in the basement of their buildings or right across the street," says Mr Parsons.

Regulators subsequently introduced new guidelines on back-up facilities and many companies have invested heavily in new operations.

According to Gregory Ferris, chairman of the Securities Industry Association (SIA) business continuity committee, another key lesson of September 11th was the need for better communication within the industry during a crisis. The SIA and other bodies set up a command centre to improve co-ordination.

"Recovery efforts following the 2003 New York City blackout and other events, such as the 2005 transit strike and national immigration rallies, were more effective because this structure was in place," he told a US congressional committee in June.

Another area of focus has been collaborative testing, such as a big exercise for the New York securities markets set for October 14th, says Anna Ewing, chief information officer of the Nasdaq stock market.

Banks and securities firms are also paying greater attention to their power and telecoms systems. Much of the disruption caused on September 11th was the result of problems with utilities, which proved unexpectedly vulnerable.