The trend, along with dramatic free-throw differentials in several of Donaghy's games, could provide important evidence to the NBA and federal authorities investigating whether the referee bet on games he officiated or furnished information to bettors as part of a possible point-shaving or game-fixing scandal.

Bookmakers pay close attention to the movement in betting lines because they often indicate that more money is being wagered on the team in whose favor the line moves. Two-point movements in NBA lines are common, and can result from legitimate strategic factors such as the absence of a key player because of injury.

It is difficult to pinpoint any possible discrepancies in Donaghy's officiating without data kept only by the NBA on how many fouls he personally called — and just as important, the players he called them against.

But games officiated by Donaghy, particularly those in which point-spread movements occurred, are under intense scrutiny as part of an FBI investigation of possible point-shaving headed by Eastern District federal prosecutors in Brooklyn. Law enforcement sources have told Newsday that Donaghy, 40, is expected to surrender to authorities as early as Tuesday or Wednesday.

The New York Knicks were involved in at least two of the games that raised a red flag for the gambling source, who spoke on condition of anonymity because of the sensitivity of the investigation.

When New York hosted the Miami Heat on Feb. 26 this year, the opening line favored the Knicks by 3 and moved to 4.5. The Knicks shot 31 more free throws than Miami (39-8) and won by 6, 99-93.

On Jan. 2, 2006, Donaghy officiated the Phoenix Suns-Knicks game at Madison Square Garden in which the opening line favored the Suns by 6.5 and closed at 6. The Knicks enjoyed a 54-16 free-throw advantage — the largest of any game Donaghy officiated in the past two seasons — and won, 140-133, in triple overtime.

Those were among 16 games Donaghy officiated in 2005-06 and '06-07 in which the home team shot at least 14 more free throws than the visitors, the source said. In five of those games, the home team shot at least 20 more free throws. Donaghy also officiated two games in which the visiting team enjoyed a free-throw advantage of 20 or more.

When the Memphis Grizzlies played at Utah on Nov. 19, 2005, the Grizzlies initially were favored by 5. The line increased to 7, and Memphis shot 19 more free throws (30-11) than the Jazz in an 80-72 victory.

On Dec. 18, 2006, the Washington Wizards opened as 2 1/2-point favorites at Denver. The line increased to 3 1/2 points, largely because of suspensions for Carmelo Anthony and Nene. The undermanned Nuggets shot 44 free throws compared with Washington's 18 and won easily, 117-108.

Of the eight playoff games Donaghy officiated in the past two seasons, two raised red flags for the Nevada betting source.

On April 28, 2006, the line favoring the host Los Angeles Lakers over the Suns moved a half-point in L.A.'s favor. The Lakers won and covered, 99-92, enjoying a 23-17 free-throw advantage with 22 fouls called on Phoenix and 14 on the Lakers. Four of the Suns' five starters finished with at least four fouls.

On April 29, 2007, the result was flipped. The Suns opened as a 4 1/2-point favorite, the line increased to 6, and Phoenix won by 13 — 113-100 — with a 31-17 free-throw advantage.

But several games in the Donaghy file don't fit the profile of a fixer, the source said, highlighting how tedious it will be for the NBA and federal authorities to build their case against him.

For example, the Philadelphia 76ers were favored to beat the Toronto Raptors by one point at home on Jan. 15, 2007, but heavy betting on Toronto resulted in the closing line favoring the Raptors by 1. Toronto won in a blowout, 104-86, and 21 fouls were called on each team.

Two nights later, heavy wagering on Phoenix for their game at Houston increased the point-spread favoring the Suns from 4.5 to 8. The Suns won, 100-91, but each team shot only 12 free throws. Rocket stars Yao Ming and Tracy McGrady missed the game with injuries, accounting for much of the betting line movement.

Another anomaly: In Game 6 of the 2007 first-round series between the New Jersey Nets and Raptors, the line opened at 3 points favoring the Nets at home and closed at 5. That result would have been under scrutiny, but only by any fixers who might have been involved. The Nets failed to cover in a 98-97 series-clinching victory, with only a slight free-throw advantage (29-25).