It has been a bad 10 months for Jair Bolsonaro.

He lost re-election as Brazil’s president. Thousands of his supporters stormed Brazil’s halls of power. And he was blocked from holding elected office for seven years.

Now things could soon get worse: Across Brazil, both his critics and supporters speculate that the next twist might be his arrest.

Mr. Bolsonaro, 68, has become ensnared in a series of investigations into fraud and election tampering that have already landed some of his closest allies in jail and that, over the past several weeks, appear to be closing in on him.

But one case may pose the biggest threat to the former president in the near term, and it revolves around an alleged scheme that resembles a small-scale mafia scam: Selling embezzled watches at a shopping mall outside Philadelphia.

This month, Brazilian federal police carried out raids as part of an investigation into what it says was a broad conspiracy by Mr. Bolsonaro and several allies to embezzle expensive gifts he received as president from Saudi Arabia and other countries. In one case, authorities accuse Mr. Bolsonaro’s personal aide of selling a diamond Rolex watch and a Patek Philippe watch to a jewelry shop at the Willow Grove Park mall in Pennsylvania last year.

Mr. Bolsonaro ultimately received at least some of the $68,000 from the sale in cash, federal police officials said.

In an interview, Mr. Bolsonaro’s lawyer, Paulo Cunha Bueno, said that whether Mr. Bolsonaro attempted to sell the diplomatic gifts is irrelevant because a government panel had previously ruled that much of the jewelry is Mr. Bolsonaro’s personal property, not the state’s. “It’s his right,” Mr. Bueno said. “It doesn’t matter.”

Some other experts in Brazilian law said that such expensive gifts are clearly state property and that Mr. Bolsonaro appeared to be in legal trouble. “To me, it seems very unlikely that the president would not be criminally charged for embezzlement,” said Miguel Reale, Brazil’s former minister of justice under a different president. Such a charge can carry penalties of up to 12 years in prison, he said. “It’s quite a delicate situation for the president.”

The case is yet another parallel between Mr. Bolsonaro and Donald J. Trump. Two far-right, nationalist leaders who attacked their nation’s democratic institutions, they both have now been accused of mishandling foreign gifts they received as president.

House Democrats have accused the Trump White House of failing to properly document more than 100 foreign gifts worth more than $250,000 combined. At the time of the House report in March, those gifts were ultimately accounted for, save for two: golf clubs from the former prime minister of Japan, Shinzo Abe, and an 8-foot-tall painting of Mr. Trump from El Salvador’s president, Nayib Bukele. Mr. Trump later said he found at least one of the golf clubs in a locker, and The New York Times found the missing painting in a back room at a Trump hotel in Miami.

Like Mr. Trump, Mr. Bolsonaro’s history with foreign gifts is hardly his only legal problem. Other investigations into Mr. Bolsonaro have heated up in recent weeks. There are probes into his possible involvement in the Jan. 8 riot in Brazil’s capital; a scheme to falsify his Covid-19 vaccine records; an alleged plot to bug a Supreme Court justice; and accusations that he ordered police to pull over his rival’s voters on Election Day. Last week, a hacker testified to Brazil’s Congress that Mr. Bolsonaro urged him to hack into the country’s election system to show it was unsafe ahead of the 2022 presidential election.

Mr. Bolsonaro denies wrongdoing in each case, saying the allegations are fabricated and political persecution. Each could carry serious criminal consequences for Mr. Bolsonaro.

Mr. Bolsonaro’s troubles with foreign gifts began in 2021 when Brazilian customs officials seized more than $3 million worth of undeclared jewels from the backpack of a Brazilian government official returning from an official visit to Saudi Arabia. The official said the jewels were a gift from Saudi officials for Mr. Bolsonaro and his wife, Michelle. Mr. Bolsonaro later made several attempts to recover the jewels, according to multiple Brazilian news outlets, including Estadão, which first reported on the seizure.

That case began a federal investigation into Mr. Bolsonaro’s handling of foreign gifts that, according to investigators, has revealed broad embezzlement and money laundering.

In one incident, police officials said, Mr. Bolsonaro’s personal aide, Lt. Col. Mauro Cid, tried to sell an 18-karat gold set from the luxury brand Chopard, including a ring, cuff links and an Arabic rosary, at a Manhattan auction house called Fortuna. In a “Valentine’s Day” auction in February, Fortuna listed the set, which the police said was a gift from the Saudi government, for $50,000, with an estimated value of up to $140,000. It did not sell.

Mr. Cid and other aides attempted to sell various other items, the police said, but only succeeded with the watches. In June 2022, while in the United States after Mr. Bolsonaro’s trip to the Summit of the Americas, Mr. Cid sold the Rolex and Patek Philippe watches to Precision Watches & Jewelry in Willow Grove, Pa., the police said.

The owner of Precision Watches said on Tuesday that the transaction was ordinary and that he had cooperated with authorities. Fortuna did not respond to a request for comment.

Brazilian law allows for presidents to keep some gifts of a personal nature, such as a custom hat, but they cannot be of high value and they specifically cannot be a valuable jewel, said Bruno Dantas, the head of Brazil’s watchdog court, the effective auditor of the federal government. “If it’s a diamond necklace with the president’s name on it, he can’t have that,” Mr. Dantas said.

To help decide, the president asks a government-appointed panel. The panel ruled that most of the jewelry Mr. Bolsonaro’s aides attempted to sell was of a personal nature.

Mr. Bueno, Mr. Bolsonaro’s lawyer, said that makes that jewelry Mr. Bolsonaro’s property. “He can sell them,” he said. “And if he dies, the assets go to his heirs.”

Mr. Dantas said the government panel erred; it should have been obvious that such expensive gifts are state property. “If this was due to incompetence, the incompetent ones will be punished for their incompetence,” Mr. Dantas said in an interview. “But if it was intentional, then you have a crime.”

Federal police have raided the home and seized the phone of the panel’s chairman, Marcelo da Silva Vieira. The judge overseeing the case has said some evidence suggests that Mr. Bolsonaro might have ordered the panel to grant him the gifts.

The chairman’s lawyer, Eduardo Kuntz, said the panel was not pressured to rule as it did and that his client “would have made the same decision a thousand times.”

Mr. Bolsonaro was still required to get separate permission from a different government body to sell the gifts, but he did not. Investigators have said Mr. Bolsonaro and his aides also tried to cover up the sales by using cash when they could or, in some cases, not disclosing the foreign gifts at all. The Rolex sold in Pennsylvania was disclosed as a gift from Saudi Arabia. But the Patek Phillipe watch was never reported, and police officials believe it came from officials in Bahrain.

When Mr. Dantas’s watchdog court learned of the jewelry this year, it ordered Mr. Bolsonaro to return it.

In March, Frederick Wassef, Mr. Bolsonaro’s former lawyer, flew to Pennsylvania and repurchased the Rolex for $49,000, the police said.

Yet last week, when asked about the Rolex, Mr. Wassef told the Brazilian news site g1: “I’ve never seen that watch.” He added, “I dare you to prove it.”

News sites then published the receipt with his name on it. Mr. Wassef admitted he repurchased the watch, but said Mr. Bolsonaro had not sent him.

The foreign jewels case, along with most of the investigations into Mr. Bolsonaro, is being overseen by Alexandre de Moraes, a Supreme Court justice who has become one of Brazil’s most powerful and polarizing figures. He has acted as the primary check on Mr. Bolsonaro’s power for years, taking ownership of most of the cases involving the former president. Last week, he authorized authorities to gain access to the foreign bank accounts of Mr. Bolsonaro and his wife.

Federal police officials also obtained Mr. Cid’s WhatsApp messages, showing his efforts to sell the jewelry and deliver cash to Mr. Bolsonaro. In one exchange on Jan. 18 with another aide to Mr. Bolsonaro, Mr. Cid said in an audio message that his father had $25,000 for the former president. “He would deliver it by hand,” he said. “The less movement in the account, the better, right?”

Mr. Cid has been in jail for months on charges that he helped falsify Mr. Bolsonaro’s vaccine records. Mr. Cid’s lawyer told reporters last week that Mr. Bolsonaro had ordered Mr. Cid to sell the jewelry.

Mr. Bolsonaro has denied receiving any money from the sales and said that Mr. Cid was acting on his own. “My brand is honesty and always will be,” he said. “There is nothing concrete against me.”

Paulo Motoryn contributed reporting from Brasília, and Ana Ionova from Rio de Janeiro.