The FBI and Miami-Dade police have opened separate criminal investigations into the campaign of a Democratic congressional candidate who, vendors say, was aided by GOP Rep. David Rivera.

Federal agents gathered campaign records, invoices and began interviewing employees at two mail and data companies used by Democrat Justin Lamar Sternad’s primary campaign. Sternad spent about $43,000 in unreported cash and checks on mail services, a witness told The Herald and authorities. Some of the money was stuffed in envelopes bulging with $100 bills.

Federal law required Sternad to quickly report any contributions — including loans —just before the Aug. 14 primary, which he lost to Democrat Joe Garcia, a longtime Rivera rival who Sternad bashed in one of his 11 mailers.

Rivera, investigators suspect, was behind the sophisticated mail campaign run by Sternad, who was an unknown political newcomer and hotel night auditor.

John Borrero, the owner of Rapid Mail and Computer Services in Hialeah, said he and his employees have met with investigators.

“I have been truthful with you, and I have been truthful with them,” said Borrero, referring all questions to police.

But Rivera strenuously denied the allegation Wednesday night in a televised interview on Spanish-language América Tevé’s (Channel 41) A Mano Limpia, insisting that he has no connection to the Sternad campaign, claiming Borrero lied and that The Miami Herald and El Nuevo Herald produced "nonsense and slander" in league with Garcia’s campaign.

“I’m going to move forward, focused on my work," Rivera said.

Rivera — who denies ever knowing Sternad — also produced a copy of a new campaign report for Sternad that purported to show, for the first time, that the Democrat’s campaign had paid Rapid Mail. Earlier in the day, a reporter for América Tevé said the congressman called her and told her to go to see Sternad’s lawyer, where she would get a scoop on the new campaign report. The lawyer never gave it to her.

Rivera didn’t explain how he got the report and Federal Elections Commission officials told The Miami Herald and El Nuevo Herald on Wednesday that the document had not been received yet by the FEC.

Even if Sternad amended his report, however, it might not be enough to avoid more questions from investigators.

Sternad referred all calls to attorney Enrique “Rick” Yabor, who refused to comment.

Borrero said he was paid in cash for all of his work, which involved scanning mail addresses on fliers. He was only paid in check from a company called Expert Printing, which paid at least $9,000.

Asked about working on Sternad’s behalf, Expert Printing’s Enrique Barrios wouldn’t talk.

“We have no comment on that,” Barrios said repeatedly. His company has handled $226,000 worth of business from Rivera over the past decade. He and a business partner have also been Rivera contributors.

Another vendor, Campaign Data’s Hugh Cochran, told The Herald that Rivera helped target the voters mailed by Sternad’s campaign. He produced a July 29 email to Rivera and Borrero concerning the data.

Rivera’s campaign didn’t deny he received Cochran’s email about Sternad’s campaign.

“Anything Campaign Data mistakenly sent to Congressman Rivera was done so in error, which has occurred previously,” the campaign said in an email, “and without Congressman Rivera’s knowledge or consent.”