NEW YORK (AP) _ David Stern insisted allegations that former referee Tim Donaghy bet on games was an isolated case and called it the worst situation he had experienced during his tenure as commissioner.

``Mr. Donaghy is the only referee alleged to have bet on NBA games and disclosed confidential information to others,'' Stern said Tuesday during a packed news conference. ``I'll say it again, I understand this is an isolated case.''

The FBI is investigating Donaghy for allegedly betting on games he officiated during the last two seasons. He is expected to turn himself in at U.S. District Court in Brooklyn later this week or early next week, according to a person familiar with the investigation.

``We understand he's accused of betting on games in the NBA,'' Stern said. ``We're not positive it's games he worked.''

However, he said, there still was the possibility that Donaghy did gamble on games he worked and that was part of the investigation.

``I understand that he will likely be accused of providing information to others for the purpose of allowing them to profit by betting on NBA games,'' he said.

Stern called it the ``worst situation I've every experienced either as fan of the NBA, a lawyer for the NBA or commissioner of the NBA.''

The FBI first contacted the NBA on June 20 to talk about a referee alleged to be gambling on games, and the two sides met on June 21, Stern said. Donaghy resigned July 9.

``Suffice to say, we would have liked to have terminated him earlier, but our understanding was the investigation would best be aided if we did not terminate Mr. Donaghy,'' Stern said.

The June call from the FBI was not the first time Donaghy faced gambling allegations.

In January 2005, Stern said the NBA investigated a dispute between Donaghy and a West Chester, Pa., neighbour. During the investigation, there were allegations that Donaghy was gambling _ not on sports _ in Atlantic City. Stern said the league contacted every casino in Atlantic City and Las Vegas and found no evidence of gambling by Donaghy.

Authorities are examining whether Donaghy made calls to affect the point spread in games on which he or associates had wagered thousands of dollars over the past two seasons, according to a law enforcement official. The referee had a gambling problem and was approached by low-level mob associates through an acquaintance, the official said.

``We take our obligation to fans in this matter very seriously,'' Stern said. ``I can stand here today and pledge that we will do every look back possible to analyze our processes and seek the best advice possible to see if there are changes that should be made and procedures that should be implemented to continue to assure fans we're doing the best we possibly can.''

Stern said league officials observed no inappropriate behaviour by Donaghy the last two seasons.

Those familiar with the investigation and the law enforcement official spoke on condition of anonymity because they weren't authorized to discuss the ongoing probe. Stern said league officials had been following the officiating scandal that rocked a German soccer league.

``My reaction is I can't believe it's happening to us,'' Stern said.