This article is more than
1 year oldThe justice department filed 23 charges against the Republican - including wire fraud and identity theft.
The move, which builds on an earlier indictment, also accused him of lying to the Federal Election Commission.
Mr Santos has yet to comment on the developments announced on Tuesday.
"Santos allegedly led multiple additional fraudulent criminal schemes, lying to the American public in the process," FBI Assistant Director-in-Charge James Smith said in a statement.
"Anyone who attempts to violate the law as part of a political campaign will face punishment in the criminal justice system," he added.
The charges build on 13 counts the New York congressman pleaded not guilty to in May. He was accused of laundering campaign funds to pay for his personal expenses and illegally claiming unemployment benefits while he was employed.
Speaking at the time, Mr Santos accused prosecutors of mounting a political "witch hunt" against him.
The new indictment alleges that he charged more than $44,000 (£35,000) to his campaign over a period of months using credit cards belonging to contributors who were unaware they were being defrauded.
On one occasion he charged $12,000 (£9,700) to a contributor's credit card, ultimately transferring the vast majority of that money into his personal bank account, the charging document says.
Prosecutors also said he reported a series of fictions loans to qualify for support from the Republican party.
"Santos falsely inflated the campaign's reported receipts with non-existent loans and contributions that were either fabricated or stolen," Breon Peace, the United States Attorney for the Eastern District of New York, said in a statement.
"This Office will relentlessly pursue criminal charges against anyone who uses the electoral process as an opportunity to defraud the public and our government institutions."
The 35-year-old will appear in court again on 27 October.
Mr Santos was elected in 2022 after scoring an upset victory in a Democrat-leaning congressional district, touting himself as the first LGBT Republican to serve in Congress.
But he has been embroiled in several serious scandals since he took office in January.
He has been accused of lying about his college degrees and his work experience; violating campaign finance and conflict of interest laws; falsely claiming his grandparents survived the Holocaust; and creating a fake animal charity that he used to siphon away cash meant for a veteran's dying dog.
In February, House Democrats filed a resolution to expel Mr Santos, a mostly symbolic action in the Republican-controlled chamber.
He has also previously faced calls to resign from within his own party, and former House Speaker Kevin McCarthy told reporters recently that he would not support Mr Santos' bid for re-election.
The latest charges come just days after a top election aide to Mr Santos admitted she falsified some of the campaign's financial records.
Nancy Marks reported a fake $500,000 (£410,000) loan that Mr Santos claimed to have given the campaign, prosecutors said.
The false reports meant the campaign qualified for the fundraising benchmarks needed to receive financial support from the national Republican Party committee.
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