In early 2019, Senator Bob Menendez of New Jersey and his new girlfriend, Nadine Arslanian, were riding high.

He had avoided a federal bribery conviction after his trial ended with a hung jury, and the couple had begun traveling the world.

Mr. Menendez proposed to Ms. Arslanian that October in India with a grand gesture, singing “Never Enough” from “The Greatest Showman” outside the Taj Mahal. They married a year later in a small ceremony in Queens and were showered with gifts from a dozen influential friends, including the head of one of New Jersey’s largest health care systems and a lawyer who would later become the U.S. attorney for the District of New Jersey.

The senator moved into his wife’s modest split-level house in Englewood Cliffs, N.J., and they have since attended two state dinners at the White House, dining with the president of France and the prime minister of India.

But their whirlwind romance has taken a sudden dark turn.

Mr. Menendez, the 69-year-old Democratic chairman of the Senate’s Foreign Relations Committee, is under investigation by the Justice Department for the second time in less than a decade. And this time, his wife is also in prosecutors’ cross hairs.

The new inquiry, led by the U.S. attorney’s office in Manhattan, appears to be focused at least in part on the possibility that either the senator or his wife received undisclosed gifts from a company run by a friend of Ms. Menendez, and that those gifts might have been given in exchange for political favors, according to two people with knowledge of the matter and subpoenas issued in the case.

Unlike her husband, who took a seat on a New Jersey school board as a 20-year-old and rose to prominence in Hudson County’s famously sharp-elbowed political scrum, Ms. Menendez, 56, has lived a mainly private life.

She holds a master’s degree in French language and civilization from New York University, but did not work outside the home while raising her two children in Bergen County, N.J., according to court records and longtime friends. She is described by friends, acquaintances and two former lawyers in much the same way: social, smart and highly fashion-conscious.

She struggled financially after a 2005 divorce and even faced foreclosure. But by 2020, the year she and Mr. Menendez were married, she had formed an international consulting company, and her assets included bars of gold bullion then valued at as much as $250,000.

Ms. Menendez, who is represented by a Washington-based lawyer, now finds herself at the center of an investigation that carries the risk of steep criminal penalties and could alter the political playing field in New Jersey and in Washington as her husband prepares to run for a fourth term next year.

The senator’s campaign finance reports underscore the gravity of the new federal inquiry. He has spent roughly $290,000 since January in connection with the investigation, federal election records show, and last month he created a new defense fund to avoid further draining his political accounts.

The senator has said that he expected the inquiry would be “successfully closed.” His office did not respond to requests for additional comment.

A halal monopoly

The full scope of the federal inquiry, including Ms. Menendez’s role, remains unknown. But the investigation appears to focus at least in part on the couple’s connection to a 40-year-old New Jersey businessman, Wael Hana, who has known Ms. Menendez since before she started dating the senator.

She and Mr. Hana were part of a group of friends who frequently socialized at restaurants in northern New Jersey; some of them shared an affinity for Cuban cigars. Several members of the network have received subpoenas, according to a friend who was interviewed by prosecutors and lawyers representing other people in the case.

Mr. Hana began operating a start-up company, IS EG Halal, in New Jersey in 2019, and it soon became the sole entity authorized to certify that any halal food product imported into Egypt from anywhere in the world had been prepared according to Islamic law.

It was an unlikely development, given that Mr. Hana, a United States citizen born in Egypt, has said in court papers that he had no prior experience in the halal industry.

Before 2019, four companies in the United States had divvied up the work of ensuring that meat exported to Egypt was free from ingredients prohibited by Islamic law and met strict processing standards, according to the U.S. Department of Agriculture.

The decision to give authorization solely to one company did not require United States approval. But it was an abrupt pivot by the government of Egypt, and news stories at the time chronicled how the steep new fees for certification were expected to lead to higher meat prices in a country that is home to 90 million Muslims.

In November 2019, the F.B.I. searched Mr. Hana’s home and offices. Investigators seized computers, cellphones, Mr. Hana’s passport and “every single piece of paper” in the company’s headquarters, Mr. Hana’s lawyer, Lawrence S. Lustberg, said in a court filing. Mr. Hana was not charged, and Mr. Lustberg, in the filing, said that his client was told he was not a target of the investigation.

Federal prosecutors and the F.B.I. are now investigating whether the senator or Ms. Menendez received a luxury car or an apartment in Washington from Mr. Hana’s company. It is, however, unclear which, if any, of Mr. Menendez’s official acts as a senator is under scrutiny by prosecutors.

Mr. Hana’s spokeswoman, Ellen Davis, has said that he got the halal business without assistance from any U.S. public official. “Allegations about cars, apartments, cash and jewelry being provided by anyone associated with IS EG Halal to Senator Menendez or his wife at all — let alone in exchange for any kind of favorable treatment — are totally without basis,” she said in May. She declined additional comment.

The investigation has reached in other directions as well.

Prosecutors have asked for any correspondence from Ms. Menendez, Senator Menendez or a prominent New Jersey developer, Fred Daibes, about a bill that has stalled in Trenton, according to a person familiar with a subpoena issued in May to State Senator Nick Sacco who was not authorized to speak about it publicly.

The legislation would limit the height of buildings near the Palisades along the Hudson River and could have scuttled a skyscraper complex at 115 River Road in Edgewater, which Mr. Daibes has been planning to build for years.

Mr. Daibes, whose company owns the building where IS EG Halal has its headquarters in Edgewater, pleaded guilty to an unrelated federal bank-fraud charge last year; his sentencing, which is expected to result only in probation, was recently rescheduled for the fourth time.

Mr. Lustberg, who also represents Mr. Daibes in the bank-fraud case, said the new Oct. 26 sentencing date should not be interpreted to mean that his client was cooperating with prosecutors. “Fred Daibes is not cooperating with the authorities against Senator Menendez or anybody else,” Mr. Lustberg said.

A shared cause, and pancakes

Mr. and Ms. Menendez first met years ago at one of his well-known haunts — an IHOP in Union City, where he was once mayor. They began dating in late 2018.

They quickly found common cause: a desire for the United States government to formally recognize the Armenian genocide, a brutal campaign by the former Ottoman Empire that killed 1.5 million people, including several of Ms. Menendez’s relatives.

In 2019, after a decade-long effort, Mr. Menendez succeeded in getting the Senate to approve a resolution acknowledging the deaths as genocide, a step that paved the way for President Biden’s formal recognition of the massacre two years later.

“When he passed it, I had tears of joy,” Ms. Menendez said in a 2020 interview with “The Armenian Report,” an English-language podcast.

“People ask me, ‘What’s the next thing? What are you pushing him to do, backing him up to do? What would you like to see?’” she added. “For me, the one and only thing I wanted was the recognition of the Armenian genocide.”

Ms. Menendez did not respond to requests for comment. But on the podcast she explained that she was born in Lebanon to Armenian parents, who fled during the Lebanese civil war. They emigrated to the United States when she was a child, eventually settling in New York.

Court records and interviews with her former lawyers, acquaintances and longtime friends show that the years after her divorce were a time of legal tumult and financial uncertainty.

She relied mainly on alimony and child-support, and at one point picked up part-time work as a hostess at a New Jersey restaurant, said Douglas Anton, a lawyer who represented Ms. Menendez in several past legal matters, including a dispute over her ex-husband’s monthly payments and a lawsuit against an insurance company.

Mr. Anton, who dated Ms. Menendez before her relationship with Mr. Menendez began, recounted being struck by her sharp intelligence and feeling frustrated that she had not pursued a career.

“Just a smart woman,” Mr. Anton said. “Her talents were being wasted.”

In 2014, after having trouble paying the $1,897 monthly mortgage on the home she now shares with Mr. Menendez, she enrolled in a federal program that lowered her payments, court records show. By June 2019 she had failed to make even the lower payments for six months, and Freddie Mac had begun foreclosure proceedings.

Just as the foreclosure began, Ms. Menendez founded her own consulting business with the help of one of her soon-to-be-husband’s close friends. It was the start of a drastic improvement in her finances.

State records show that Ms. Menendez’s business, Strategic International Business Consultants, was incorporated on the night the mortgage company began foreclosure proceedings. The simple, one-page form creating the limited liability company was faxed to New Jersey’s Department of Treasury by a law firm run by Donald Scarinci, who testified during the senator’s bribery trial in Newark.

The company is run solely by Ms. Menendez from her home, according to state records, and does not have an online footprint or phone number. It appears on Mr. Menendez’s required financial disclosure reports because they are married, but little else about the nature of the firm is known.

Mr. Scarinci declined to comment.

The foreclosure was dismissed two months later after part of the debt was repaid and the mortgage was reinstated, according to a representative at the law firm that handled the foreclosure proceeding.

Then, a year and a half after their marriage, Mr. Menendez amended a 2020 financial disclosure form to include a new asset belonging to his wife: bars of gold bullion worth as much as $250,000.

It is unclear how she came into possession of the gold, and in late June Mr. Menendez filed yet another amended report, noting that his wife had sold between $200,000 and $400,000 of the bullion.

David Schertler, Ms. Menendez’s lawyer, declined to comment.

A wide net

Proving that elected leaders accepted gifts or favors in exchange for an official act has only gotten harder in the five years since Mr. Menendez’s 2017 federal bribery trial, after a series of Supreme Court rulings narrowed the definition of what kind of conduct can serve as the basis for a corruption prosecution. Prosecutors from the Southern District of New York have seemingly cast a wide net, issuing subpoenas in phases as the investigation broadened.

A spokesman for the Southern District declined to comment.

Despite the investigation, Mr. Menendez has steadily raised funds to run for re-election, and he has remained a vocal presence in Washington and in New Jersey.

“I know Bob to be a real decent, honest guy, who I consider to be a good friend,” said Mark J. Sokolich, the Democratic mayor of Fort Lee, N.J.

He said he did not know Ms. Menendez as well but had spent time with the couple socially.

“Very nice, very pleasant,” he said before offering an example of a typical exchange.

“‘Hey, Nadine. Hey, Mark.’ A little cordial kiss. ‘How’s the weather?’ But not halal meat, or whatever this freaking thing is about.”

Kirsten Noyes contributed research.