A failed Democratic congressional candidate whose campaign is under federal grand jury investigation abruptly amended his financial disclosures to show he loaned himself nearly $53,000 more than he originally reported.

Before filing the new report, Justin Lamar Sternad, 35, insisted his finances were in order and steadfastly refused to say how he paid for tens of thousands of dollars worth of campaign-mail services.

Many of the transactions were in cash — sometimes in the form of $100 bills stuffed in envelopes, a campaign vendor said.

The big cash payments drew the attention of the FBI, which began examining the working-class hotel employee, a political unknown who now claims to have dropped about $64,000 of his own money in his Aug. 14 primary loss.

Campaign vendors told The Miami Herald and El Nuevo Herald that Sternad’s congressional Democratic primary run was backed by Republican Rep. David Rivera, who says he has never met or helped Sternad. During the campaign, Sternad bashed Rivera’s main rival, Joe Garcia. Garcia handily won and faces Rivera in November.

Rivera and Sternad have denied working in concert. Rivera has publicly attacked Sternad’s campaign vendors who spoke about his role, calling them liars or Herald lackeys. Sternad has blamed them for his reporting problems.

“I did not previously report this loan because I was unaware of the final monetary obligation incurred by my campaign,” Sternad wrote to the Federal Election Commission, which posted his letter Thursday. “I have now received invoices for the expenditures ... and this amendment represents satisfaction of those invoices.”

The filing of an amended report doesn’t mean that law enforcement is done examining his case, say legal experts. They say it’s unlawful to knowingly and willfully file a false federal campaign report — even if it’s amended later.

John Borrero, president of Rapid Mail & Computer Services in Hialeah, told El Nuevo Herald last week that Sternad’s campaign paid about $43,000 in cash for mailings.

Some payments were allegedly made by Ana Alliegro, Sternad’s campaign consultant, totaling at least $7,000, in crisp hundred dollar bills, stuffed in envelopes, several sources familiar with the criminal probe said.

Alliegro, according to the sources, also went to Rapid Mail after the initial Herald story, published on Aug. 16, demanding all records linked to Sternad, ordering employees to throw out everything and finally telling Borrero and his staff “I hope you have a lot of clients.”

Alliegro told The Herald Thursday night that she never paid cash, and that she went to Rapid Mail solely to collect the invoices and documents associated with the Lamar mailings, so he could file his amended financial disclosure.

Borrero said he turned everything over — to law enforcement.

As a result, investigators have ramped up the probe, interviewed witnesses and subpoenaed campaign records. A federal grand jury is now involved.

Kenneth Gross, a Washington-based campaign-finance expert not connected to the case, amending a report may help him “mitigate” criminal exposure. “But it depends on the gravity of the case, the amount involved and the intent,” he said. “Was the report amended of the candidate’s own volition, or were there other pressures at play?”