WASHINGTON - When the relationship between Sen. Robert Menendez (D., N.J.) and campaign donor Salomon Melgen first made headlines, the senator admitted he goofed.
He conceded he had not paid for or properly reported four flights (two round-trips) on Melgen's private jet and wrote a check to belatedly comply with Senate ethics rules. A year later, his campaign reimbursed Melgen for a third trip. Both times Menendez aides assured reporters that these were oversights and that the senator's travel had been thoroughly reviewed.
But a federal indictment charging Menendez with corruption has revealed free flights the senator never disclosed or paid for - nine others for him, sometimes with a guest - in apparent violation of Senate ethics rules.
While breaking those rules once typically results in no more than a Senate scolding, Menendez's failure to report the free trips over several years could now be used as damning evidence as he faces 14 counts of federal corruption, including some that carry penalties of up to 20 years in prison.
"Often what a prosecutor will say is this is evidence of willful intent - he was hiding this because he knew this was wrong," said Meredith McGehee, policy director of the nonpartisan ethics watchdog the Campaign Legal Center.
"Usually it's like, 'Bad boy, next time, disclose.' It's usually of that nature," McGehee said. But in a bribery trial, "this is kind of a whole different nature."
The question of intent is vital to the prosecutors' case as they try to prove a criminal scheme in which Melgen allegedly showered Menendez with gifts, and the senator, in turn, pressed federal officials to help his friend's financial, business, and personal interests.
Menendez - who is scheduled to appear on Fox News Sunday and who is due back in the Capitol on Monday for the first votes since his indictment - has said he followed the law and would be vindicated. He has said he and Melgen, a South Florida eye doctor, are friends who have long exchanged gifts, as friends do.
To overcome that defense and the absence of an explicit quid pro quo agreement, prosecutors will have to show a criminal motive behind the gifts. They'll likely rely on Menendez's repeated lack of disclosure to help them, legal experts said.
"The prosecutors are going to attempt to make a whole lot of that point," said Lee Vartan, a former federal prosecutor in New Jersey, now with the law firm Holland & Knight. Predicting prosecutors' argument, he said, "If this was all appropriate, then why wasn't Sen. Menendez making the required ethics disclosures?"
Menendez's travel on Melgen's plane drew widespread attention in January 2013, when federal agents raided Melgen's South Florida medical practice.
Menendez publicly admitted then he had written a $58,500 check to Melgen weeks before the raid - and two months after a Republican ethics complaint - to cover two round-trips in 2010 that he had not disclosed.
They included flights on Aug. 6, Aug. 9, Sept. 3, and Sept. 6 that year. In each case, Menendez flew to the Dominican Republic, where Melgen has a vacation home, and then to New Jersey.
Menendez's office blamed the failure to pay or report the flights on "sloppy" paperwork. A spokesman at the time, Paul Brubaker, said that the flights had turned up as part of an "exhaustive review" following the ethics complaint and that there were "no outstanding issues" with Menendez's travel.
Menendez's chief-of-staff at the time, Dan O'Brien, told NBC News that there was only one other trip on Melgen's plane in 2010 and that it had been covered by a Democratic campaign fund. That flight and at least one other covered with campaign funds were not mentioned in the indictment.
The indictment, though, says that in 2010, Melgen paid $890.70 for a first-class ticket for Menendez to fly from New Jersey to West Palm Beach, Fla., near Melgen's home. Days later, he paid $8,037 to charter a private flight for Menendez to fly from Florida to Washington.
Earlier that year, Melgen's jet flew an unidentified Menendez guest to meet the senator at Melgen's Dominican Republic home, then returned the guest to Florida days later, the indictment alleges.
In all, in 2006, 2007, 2008, and 2010, prosecutors laid out 10 flights that Menendez took on Melgen's private jet, plus three the senator took on other aircraft that Melgen paid for or arranged.
Melgen also sent his jet five more times to pick up Menendez at the start of trips, the indictment says.
Menendez's 2013 payment covered four of the flights listed in the charges. The indictment does not note this payment.
Asked last week about the flights Menendez did not disclose or pay for after his travel became a point of public concern, his office declined to comment.
Under Senate ethics rules, senators must receive written approval to accept a gift from a friend worth more than $250. Menendez did not do so.
And though the rules allow a "personal hospitality" exemption for a pleasure ride on a friend's plane or boat, the rides cannot be used as a substitute for commercial travel.
Once the gifts received in a given year reach a certain value, senators must report them on their annual financial-disclosure forms. In the years of the Melgen flights, the reporting threshold was between $305 and $335, the indictment says.
It also alleges that the failure to report the flights and other gifts "was part of the scheme to conceal that Menendez received things of value from Melgen."
The last of the counts against the senator charges him with "false statements."
It is among the lightest in the case - at most it carries a penalty of five years in prison and a $250,000 fine. Alone, it probably wouldn't be the main thrust of a high-profile indictment, former prosecutors said.
Nondisclosure cases are generally the easiest to make, said Andrew Levchuk, a former federal corruption prosecutor. Now with the firm Bulkley, Richardson & Gelinas in Massachusetts, he said he was "very surprised" at the lack of disclosure given the volume of gifts alleged.
"Those disclosure regulations and rules are the linchpin of people keeping an eye on Congress," Levchuk said. "I disagree with people who take the point of view that a false statement charge is not a big deal. To me, it's a big deal."