Judge William Walls ordered the seven-woman, five-man jury home for the day to "clear their heads" and return Tuesday morning at 9:30 a.m. ET.
It was a dramatic turn of events in a trial now entering its eleventh week.
The New Jersey Democrat faces charges of conspiracy, bribery and honest services fraud related to abusing the power of his office. Prosecutors say the senator accepted more than $600,000 in political contributions, a luxurious hotel suite at the Park Hyatt in Paris and free rides on a private jet from a wealthy ophthalmologist, Dr. Salomon Melgen, in exchange for political favors.
Last week, a dismissed juror predicted a hung jury.
Jurors' struggle to reach a unanimous decision could signal a potential mistrial down the line, a fitting coda for a trial with often conflicting, muddled testimony.
The defense team had maintained throughout the case that Menendez and Melgen were longtime friends with no corrupt intent to commit a federal crime, and after hours of testimony and hundreds of exhibits, the prosecutors never produced a smoking gun in the form of a document or phone call outlining an illicit agreement between the two men.
Instead, the case rested largely on circumstantial evidence. Emails and testimony revealed the senator's efforts to encourage State Department officials to favorably resolve a contract dispute Melgen had with the Dominican Republic over cargo scanning equipment, but also showed he was genuinely interested in port security issues. Similarly, witnesses told jurors that Menendez wanted officials in the Obama administration to change Medicare billing policies to help Melgen resolve a multimillion-dollar overbilling dispute, but also learned that he was concerned the drug companies were enjoying a windfall from the current policies.
Many witnesses simply couldn't pinpoint exactly what Menendez's ask of them was once they took the witness stand.
"I don't exactly know what he wanted other than for me to do something," former Health and Human Services Secretary Kathleen Sebelius told jurors, describing a 2012 meeting with Menendez that was arranged by then-Senate Majority Leader Harry Reid, a Nevada Democrat, and involved Menendez's "unhappiness that there was a CMS policy that Sen. Menendez felt was unclear and unfair to providers."
But neither Sebelius, nor a former high-level official at the Centers for Medicare and Medicaid Services could recall Menendez directly mentioning Melgen's name in their interactions with the senator.
"I got in the meeting, he brought up the (billing) policy, asked me to take a look at it," former CMS Administrator Marilyn Tavenner testified. "So, it kind of connected those dots for me."
'You are starting fresh'
Their 16 hours of deliberations -- which spilled into rare public view
last week -- continued from scratch Monday morning with an alternate juror replacing one who was excused due to vacation plans.
"You are starting fresh," Walls told the jury. "Forget about what happened last week. This is the jury."
In an unusual tell-all to CNN and other news outlets last week, Evelyn Arroyo-Maultsby, formerly juror No. 8, revealed the divisions among jurors over the multi-count indictment Menendez is facing and suggested "it's going to be a hung jury."
"(Prosecutors) just didn't show me enough, and I just wish I wasn't going on vacation," Arroyo-Maultsby said. "I would've been fighting in that jury room."
Headed into court Monday, Menendez told reporters that he "wished she hadn't left."
"I wish her a great vacation, I wish she hadn't left, but I hope that no juror in the jury room feels bullied by another juror," Menendez said. "I'm concerned about some of the comments she made ... the environment in the jury room."
Over the objection of the prosecution, Walls questioned jurors about whether they heard or read Arroyo-Maultsby's comments to the press. Four jurors and, later, three alternates slowly raised their hands, but ultimately none were dismissed after further questioning behind closed doors with the judge and attorneys.
Menendez's defense attorney, Abbe Lowell, urged Walls to engage in a more expansive inquiry regarding reports Arroyo-Maultsby wanted to contact the judge about deliberations last week and felt certain jurors were trying to "run out the clock" on her, knowing she had to leave on a pre-scheduled vacation.
"That's life that they ran the clock out on her," Walls told attorneys, unpersuaded that any harm was done. "She never had the votes."
Headed for mistrial?
Former federal prosecutor Lee Vartan said he'd be "sweating" right now if he were still at DOJ.
"All of this is unique and atypical," Vartan said, but "as a prosecutor, you don't want this sort of feedback from someone who heard all of the evidence. It's about the worst thing that you could hear as a prosecutor."
If the jury officially deadlocks in a mistrial or acquits Menendez of all charges this week, the senator won't be the only one breathing a sigh of relief.
Menendez's trial is being closely watched for any impact on the balance of power in the US Senate, where Republicans have a slim 52-48 majority and Democrats want to hold onto the seat. If he's cleared of all wrongdoing, or at the very least, able to keep sentencing at bay until after January 16, 2018 -- when GOP Gov. Chris Christie leaves office -- Republicans would lose any leverage to push for his ouster.
A mistrial wouldn't necessarily mean Menendez will be off the hook for good, however.
Should Walls formally declare a mistrial, former federal prosecutors predict that the Justice Department would ultimately refile the charges.
"If you believed in the case enough to bring it in the first place, typically you bring it again," said Randall Eliason, a law professor at George Washington University, who has followed the trial closely.
Though some say prosecutors might streamline the presentation a second time around.
"The evidence at trial should have been presented in more surgical and direct manner," said Michael Weinstein, a former trial attorney at DOJ and now at Cole Schotz law firm in New Jersey. "The goal of doing so should have been to clearly and sufficiently tie together the hundreds of pieces of evidence into a more cohesive story. It would exceptionally unusual for them to walk away from this type of public corruption case."