(Adds excerpts from Christie's ABC interview)
By Barbara Goldberg and Victoria Cavaliere
NEW YORK, March 27 (Reuters) - A law firm hired by New Jersey Governor Chris Christie to investigate the "Bridgegate" scandal exonerated the potential Republican presidential contender on Thursday in a report quickly dismissed by critics as whitewash.
The review cleared every member of Christie's current staff, but blamed former members of his inner circle whom he fired soon after a scandal erupted over the September 2013 shutdown of traffic lanes leading to the George Washington Bridge in Fort Lee, New Jersey.
"What we found was that Governor Christie had no involvement in the decision to close these lanes and no prior knowledge of it," said attorney Randy Mastro of the law firm Gibson, Dunn & Crutcher, which conducted the investigation.
Two key players who orchestrated the massive traffic jam were Bridget Anne Kelly, the governor's former deputy chief of staff, and David Wildstein, a Christie appointee to the Port Authority of New York and New Jersey, which oversees the bridge, according to the report.
Their motive was to punish Fort Lee Mayor Mark Sokolich, a Democrat whom Wildstein did not hold in high regard, Mastro told a press conference in New York. Fort Lee sits at one end of the George Washington Bridge, the nation's busiest span, and the lane closures caused massive backups in the borough.
Results of the 10-week probe were met with skepticism by New Jersey Democrats, who have commissioned a bi-partisan panel to investigate the lane closures.
"Lawyers hired by and paid by the Christie administration itself to investigate the governor's office, who then say the governor and most of his office did nothing wrong, will not be the final word on this matter," said a statement from Assemblyman John Wisniewski and Senate Majority Leader Loretta Weinberg.
Aside from the state investigation, a federal probe is under way by U.S. Attorney for New Jersey Paul Fishman.
Christie has adamantly denied any prior knowledge of the plan, and in a pre-taped interview set to air on ABC late Thursday, the governor again said he was shocked by the actions of his former aides.
"Sometimes, people do inexplicably stupid things," Christie said in excerpts released prior to the full interview.
Christie's investigators failed to gain access to at least four central figures, including Kelly and Wildstein, accused of either designing or covering up the scheme.
While Mastro said the review was "costly" for New Jersey taxpayers, he declined to confirm a New York Times report that put the price at $1 million.
Critics dismissed the probe as an effort to shore up Christie's tarnished image ahead of a possible White House run in 2016.
"A very elegant and expensive whitewash" said Ross Baker, a political science professor at Rutgers University, the state university of New Jersey.
The 344-page report said the investigation involved 250,000 documents and 70 interviews, but excluded some of the main players in the scandal, such as Wildstein, Kelly and Bill Stepien, Christie's former campaign manager.
Mastro defended the report as thorough and objective.
"Witnesses lie, documents don't. We have hard evidence," Mastro said. "If we had found evidence to the contrary, we would have reported that."
The report also doubted the veracity of a conversation between Wildstein and Christie's press secretary after the scandal broke in December, in which Wildstein said he had informed the governor of the traffic problems.
Instead, the report said, blame lies chiefly with Kelly, who in August sent Wildstein an email saying "Time for some traffic problems in Fort Lee", and Wildstein who emailed back: "Got it."
Fort Lee's Mayor Sokolich had declined to endorse Christie's re-election bid, and some have speculated the lane shutdown was political payback.
But Mastro said the investigation confirmed only that Wildstein approached Kelly with the plan and there appeared to be "a focus and animus toward Sokolich not explicitly tied to his endorsement."
Also implicated were Stepien and Bill Baroni, then deputy executive director of the Port Authority.
"(Stepien and Baroni)knew of this idea in advance, but we found no evidence that they knew of the ulterior motive here," the report said.
Lawyers for Kelly, Wildstein and Baroni did not immediately comment and Stepien's lawyer said the report cleared him of wrongdoing.
The report also denounces as lies the allegations by Hoboken Mayor Dawn Zimmer, who charged that Lieutenant Governor Kim Guadagno warned her that Christie would withhold Superstorm Sandy recovery funding if Zimmer did not support a development project in her city. Guadagno has denied the accusations.
Zimmer said in a statement on Thursday it was "sadly predictable" that the report said her claims were false. (Additional reporting by Jonathan Allen in New York and David Jones in New Jersey; Editing by Gunna Dickson)