Midway through opening arguments in the trial of former allies to Gov. Christie on charges they illegally orchestrated a massive traffic tie-up at the George Washington Bridge in 2013, Assistant U.S. Attorney Vikas Khanna dropped a potential bombshell:

Confidants had boasted to Christie at a 9/11 ceremony of the traffic snarl in Fort Lee as it was unfolding, and that they were not returning the panicked phone calls of the Fort Lee mayor.

The surprise disclosure, which marked the first time federal prosecutors suggested that Christie may have known of the lane closings as they occurred, prompted questions on why the governor himself had not been charged.

But experts in criminal law say that investigations of this kind often turn on nuances and that Khanna's language was couched. Even if federal prosecutors have evidence that Christie knew about the scheme, it might not be evidence of a crime, they say. At the same time, the Justice Department's U.S. Attorney's manual sets stringent rules, barring criminal charges unless prosecutors are convinced that they can prevail at trial.

One witness with a story to tell typically isn't enough in such cases, the experts say.

"There are two issues: First of all there is the legal question of whether mere knowledge would be enough to support criminal charges, even if you could prove knowledge," said Robert Mintz, head of the white-collar defense practice at McCarter & English, a prominent, Newark, N.J.-based firm with an office in Philadelphia. "And the second issue is the proof issue. Federal prosecutors rarely bring charges based on one individual's uncorroborated testimony. They need more than that. It can't be just one person testifying that something happened. They would typically need corroboration."

Simply being aware of a crime isn't in itself a criminal act, except in cases such as child or elder abuse, where the law requires that such crimes be reported.

"Just because you know of a crime doesn't mean you are an accomplice; that is pretty strict black-letter law," said Thomas Bergstrom, a white-collar defense lawyer at Buchanan Ingersoll & Rooney in Center City.

Brian Murray, Christie's press secretary, declined to comment on the trial except to point to an earlier statement by the governor that he had no recollection of the conversation with David Wildstein and Bill Baroni. On other occasions, the governor has said he had no knowledge of the plan to cause the traffic tie-ups.

According to Khanna, there were three people involved in the Sept. 11, 2013, conversation: Wildstein, a onetime Christie ally who is a federal cooperating witness; Baroni, the former deputy executive director of the Port Authority of New York and New Jersey, which operates the bridge; and the governor. Baroni has been charged with fraud and conspiracy along with former Christie administration official Bridget Anne Kelly in connection with orchestrating lane closings on the eastbound side of the bridge from Sept. 9, 2013, through Sept. 13, causing the traffic troubles.

Khanna's mention of that meeting was brief, and open to interpretation.

"The evidence will show that Baroni and Wildstein were so committed to their plan ... that during the precious few minutes they had alone with the governor, they bragged about the fact that there were traffic problems in Fort Lee and that Mayor Sokolich was not getting his calls returned," Khanna said in his opening statement.

Wildstein, Kelly, and Baroni allegedly arranged the lane closings as retribution for the failure of Fort Lee Mayor Mark Sokolich, a Democrat, to endorse the reelection campaign of Christie, a Republican.

It was a particularly vicious tactic, federal prosecutors assert. The lane closings were timed to coincide with the first day of school to maximize the disruption and discomfort. Sokolich called Baroni and officials in the governor's office, at one point reporting that the traffic jam had interfered with a police search for a 4-year-old child and the emergency response to a man suffering from a heart attack. But Baroni declined to respond.

Steven Feldman, a former federal prosecutor in the U.S. Attorney's Office in Manhattan who now practices as a white-collar defense lawyer at Murphy & McGonigle in New York, said the government may have had strategic reasons for mentioning the meeting with Christie. If Wildstein told federal prosecutors the governor knew about the plot, they would be obligated to turn over that information to defense counsel, who could use it to make the point that their clients were merely minor players in a larger scheme.

Flagging that at the outset might help to deflate the defense argument that Christie was in on the scheme.

"[Prosecutors] know what the cooperating witness is going to say, and that the defense will use that," Feldman said. "How do you approach that? Oftentimes, you want to take some of the sting out of it by mentioning it yourself."

Either way, it won't be an easy case for the government, said Patrick J. Egan, a criminal-defense lawyer at Fox Rothschild in Center City. That's because the usual elements of a political corruption case – the exchange of money for some favorable government action --  are missing, he said.

"It's not the typical criminal conduct that you see in court every day," Egan said. "It's not a situation where someone is trying to enrich themselves. Usually you have someone giving something to a politician and getting something in return. Here you have a bridge closing."