Another online brokerage fell victim to an apparent intentional attack Thursday, vanishing from the Internet for at least an hour.

National Discount Brokerage Group

( NDB) said it lost an undetermined amount of business when its Web site, the company's main pipeline for fee-based trading service, shut down completely around 1 p.m EST.

The company said it had notified the

Securities and Exchange Commission

, the

National Association of Securities Dealers

and the

Federal Bureau of Investigation

. The FBI is investigating "denial of service" attacks, typically executed by bombarding a Web site with a wave of false information requests from a conglomeration of computers.

The attacks drew scrutiny to Internet security earlier this month, when hackers shut down the popular site of several big Internet companies including

Yahoo!

(YHOO)

,

eBay

(EBAY) - Get Report

and

E*Trade

( EGRP), another online brokerage.

Dennis Marino, chairman of National Discount Brokers, said the firm saw that its Web site was operating haltingly early Thursday morning. An internal investigation showed an abundance of incoming traffic from two Internet protocol addresses, which Marino declined to identify. The FBI has requested the results of the investigation.

It was not until the company tried to filter out the problematic traffic, around 1 p.m., that the system crashed, disengaging the Web site from both

PSINet

(PSIX)

and

Exodus

( EXDS), the two Internet service providers used by the company. Service was lost for at least an hour, perhaps 1 1/2 hours, Marino said.

"It's not been one of our better days," Marino said, adding that he would not venture to guess how much money the company lost on commissions.

The FBI has said additional Web sites have suffered similar attacks, but the companies involved have not disclosed these problems, as National Discount Brokers did.

"We're a public company," Marino said. "We were out of service today. Our customers and our shareholders ought to have an understanding of what happened. This is certainly not something that's unique to us."

Marino said some customers continued trading using the company's touch-tone telephone service and placing orders with brokers on the telephone "the old-fashioned way."