Did fake donors really give the mayor money? The FBI wants to know.

It’s the kind of scheme that toppled New York’s lieutenant governor last year and sank the 2013 mayoral campaign of a top Democratic candidate: using so-called straw donors to funnel illegal contributions from secret sources to candidates.

Now for the second time, Mayor Eric Adams’ campaign is under scrutiny on the same issue.

On Thursday, the FBI searched the home of Brianna Suggs, Mr. Adams’s top fundraiser, as part of an investigation into whether his campaign had received illegal foreign campaign contributions from the Turkish government and Turkish nationals disguised as coming from American donors who had did not actually give them their own money, according to a search warrant.

And in July, six men were indicted in Manhattan in connection with a similar scheme, accused of funneling thousands to Mr. Adams’ campaign. Two brothers do has pleaded guilty to one count of conspiracy in the case, and The City news channel and other organizations have done so found even more inconsistencies in donations to the mayor’s campaign.

Neither Mr. Adams nor Ms. Suggs have been accused of wrongdoing, and Mr. Adams has denied any knowledge of illegal contributions. But both investigations appear to focus on whether donors eager to get Mr. Adams’ attention tried to mask large donations by channeling them through straw donors — and who might have coordinated that effort.

The investigation also raises questions about whether Mr. Adams properly vetted his donations to root out abuse. Andrew Yang’s rival 2021 mayoral campaign netted two staff members donations from veterinarians of more than $100, according to a person familiar with the matter.

A lawyer for Mr. Adams’ 2021 campaign, Vito Pitta, said in an email that the campaign had worked to rigorously identify and investigate questionable contributions. Mr. Pitta said the campaign has received more than 10,000 donations and has been working to match handwriting and signatures, review donor confirmation forms and more.

In New York City, small donations are particularly attractive. A generous matching program provides $8 in public funds for every $1 donated. That changes the maximum donation from $250 to $2,250, a potential temptation for anyone looking to multiply their money.

“That’s probably the dumbest way to waste money trying to influence a candidate,” said John Kaehny, executive director of the government watchdog group Reinvent Albany. “They are all being looked at by the Campaign Finance Board, which has the most comprehensive review and audit process in the United States. I think it’s actually a sign of amateurism.”

The city’s Campaign Finance Board is scrutinizing the donations and has reviewed donations to Mr. Adams’ first mayoral campaign.

Mr. Adams, a moderate Democrat and former police chief, has been involved in politics for decades, and his fundraising tactics have repeatedly pushed the boundaries of campaign finance laws and ethics.

As a state senator, he became embroiled in a scandal after a committee he led helped choose a supplier of video lottery machines at Aqueduct Racetrack.

And according to an indictment in the Manhattan case, a retired police inspector who worked with Mr. Adams and interacted with a potential donor told Mr. Adams “he doesn’t want to do anything if he doesn’t get 25 Gs” — a reference to the minimum of $ 25,000 that he expected for attending a fundraising event.

The search of Ms. Suggs’ Brooklyn home was part of a broad public corruption investigation. Ms. Suggs, 25, is in the mayor’s inner circle and has close ties to Ingrid Lewis-Martin, Mr. Adams’s closest aide and confidante, who has played an active role in his and others’ campaigns.

The order suggested that foreigners had made campaign contributions through a straw donor program.

The warrant authorized agents to seize evidence relating to payments or reimbursements made to employees of KSK Construction Group in Brooklyn, “or other persons serving as conduits for campaign contributions to the Adams campaign originating from Turkish nationals.”

City campaign financials reflect contributions to Mr.’s first mayoral campaign. Adams from 11 KSK employees, all on May 7, 2021, for a total of nearly $14,000. Nine of the eleven had the same amount of $1,250; most were eligible to earn a matching amount for the campaign, the data shows.

Ten years ago, John Liu, then the city comptroller, was a leading contender for mayor when an investigation into his campaign uncovered a scheme to funnel money through straw donors.

Two of his former associates were convicted in the scheme in 2013, including Jia Hou, a former Liu campaign treasurer who was in his 20s. Mr. Liu, who is now a senator representing a district in Queens, was not indicted, but his mayoral campaign never recovered and he finished fourth in the Democratic primary.

Last year, Brian Benjamin, then New York’s lieutenant governor, resigned after being indicted in what federal prosecutors described as a brazen scheme that appeared to involve straw donors.

Mr. Benjamin was accused of accepting thousands of dollars in illegal donations from a developer for his 2020 Senate campaign and his failed 2021 bid for New York City comptroller, the indictment said. (A federal judge later dismissed the bribery charges against Mr. Benjamin, but left unchanged two charges of falsifying records related to straw donations.)

Chris Coffey, a Democratic political strategist and Yang’s campaign manager, said donors did not always understand that it was illegal to contribute money in someone else’s name.

“Donors are often unaware of campaign finance issues,” he said. “They think it’s like donating to a charity for which you can get compensation. With every campaign, the pressure increases to make sure people know the rules.”

Whether Mr. Adams knew details about potential straw donors to his campaign has given the appearance that he is open to influence, said Mr. Kaehny of Reinvent Albany.

“There is great concern that the city is for sale and that New York has returned to the bad old days, where pay to play and bribery were just part of political life,” he said.