The NBA acknowledged Friday the FBI is investigating Tim Donaghy for betting on games, including ones in which he officiated.

"It's a shame," the Los Angeles Lakers' Kobe Bryant said. "It's just unfortunate. ... Like I said, it's a very serious issue."

According to a law enforcement official, authorities are examining whether the referee made calls to affect the point spread in games on which he or associates had wagered thousands of dollars over the past two seasons.

The referee had a gambling problem and was approached by low-level mob associates through an acquaintance, said the official, who spoke to the AP on condition of anonymity because he wasn't authorized to discuss the ongoing investigation.

Gambling long has been a problem in sports, and leagues have made a point of educating players of the potential pitfalls. The NBA, for example, discusses gambling at rookie orientation, even bringing in former mobster Michael Franceze to speak.

And the NBA dealt with negative stories about its officials earlier this year when an academic study detailed a bias by referees against players of the opposite color. The league requires its officials to file reports and defend or discuss every questionable call they make in a game.

Donaghy, who reportedly has resigned, was an NBA official for 13 years. He officiated 68 games in the 2005-06 season and 63 games in 2006-07, according to the Elias Sports Bureau. He also worked 20 playoff games, including five last season _ Pistons-Magic on April 23; Warriors-Mavericks on April 27; Suns-Lakers on April 29; Nets-Raptors on May 4; and Spurs-Suns on May 12.

Those studying Donaghy's games might have noticed some trends.

When the home team was favored by 0-4 1/2 points, it went 5-12 against the spread in games officiated by Donaghy this season, according to Covers.com, a Web site that tracks referee trends. Home underdogs were 1-7 against the spread when it was 5-9.5 points.

Donaghy was part of a crew working the Heat-Knicks game in New York in February when the Knicks shot 39 free throws to the Heat's eight, technical fouls were called on Heat coach Pat Riley and assistant Ron Rothstein, and the Knicks won by six. New York was favored by 4 1/2.

Defense attorney John Lauro confirmed Donaghy is under investigation, but refused to comment on the allegations or the case.

"They are serious allegations," the leader of the referees' union, Lamell McMorris told the Philadelphia Inquirer. "It's an ongoing federal investigation. And we don't really have much more to say about it, and neither do the referees."

In Bradenton, Fla., a woman came to the door of the home where Donaghy lives and shouted through the door: "We have no comment."

Donaghy is perhaps best-known previously as one of the referees in the 2004 game at Detroit that ended with Indiana Pacers players fighting with Pistons fans, among the biggest black marks in league history.

This could top it.

"We would like to assure our fans that no amount of effort, time or personnel is being spared to assist in this investigation, to bring to justice an individual who has betrayed the most sacred trust in professional sports, and to take the necessary steps to protect against this ever happening again," commissioner David Stern said in a statement.

Stern's statement said the FBI is investigating allegations a "single" referee bet on basketball. But the law enforcement official said other arrests are expected.

NBA players in Las Vegas for USA Basketball minicamp were surprised and disappointed to learn of the accusations.

Denver Nuggets forward Carmelo Anthony said he was confident the NBA would sort it out.

"I'm pretty sure they're going to take care of it when they find out what's going on, so I leave it up to them," he said. "It's shocking. I'm pretty sure they're going to get to the bottom of it."

Nevada gambling regulators were not involved in an investigation and had no information about the allegations, said Jerry Markling, enforcement chief for the state Gaming Commission and Gaming Control Board.

Veteran oddsmaker John Avello, at the Wynn resort on the Las Vegas Strip, said that without specific information it would be difficult to identify wagering irregularities over the last two seasons.

"At this point, it's too early to know if any games were affected," Avello said.

The law enforcement official said the referee was aware of the investigation and had made arrangements to surrender as early as next week to face charges.

The investigation was first reported Friday by the New York Post.

"I'm shocked, terribly shocked," said Gary Benson, an NBA official for 17 years who retired two years ago.

Benson said he didn't work with Donaghy much.

"You have a lot of acquaintances and very few friends. ... I probably worked a handful of games with him overall, just a handful."

Donaghy's neighbors in Bradenton also knew little about the man who has grabbed the attention of the NBA and FBI.

Bob Girard, who lives near Donaghy in a gated community along a golf course, said he only noticed one thing out of the ordinary about his neighbor.

"His house just went up for sale," said Girard, who recalled Donaghy moving into the neighborhood less than a year ago.

When Girard saw the news of the NBA betting scandal on TV, he wondered whether it might involve his neighbor, the NBA referee with daughters who sometimes sold lemonade in front of their house for five cents a cup.

"They've got a nice family," Girard said. "They seem to be a pretty normal family to me."

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Associated Press Writer Pat Milton in New York, AP Writer Ken Ritter in Las Vegas, AP Sports Writers Noah Trister in Little Rock, Ark., and Fred Goodall in Bradenton, Fla., contributed to this report.