A key witness in a federal grand jury case involving U.S. Rep.David Rivera is still missing, but she left important evidence behind for investigators: at least four envelopes that had been stuffed with unreported campaign cash.

Ana Alliegro, a Republican political operative, delivered the cash-stuffed envelopes to a Hialeah mail house that sent out fliers in a congressional race against a Rivera political rival, the mail house owner told the FBI.

The FBI has the envelopes to check for fingerprints and handwriting comparisons.

Also in the hands of FBI agents: at least six invoices initially made out to the attention of David Rivera — all marked paid “cash” — to cover the mailings for Democratic primary challenger Justin Lamar Sternad, a suspected Rivera straw-man candidate. The congressman demanded that his name be removed from the invoices with Wite-Out, documents and interviews show.

Alliegro went missing two weeks ago, shortly after her computer was seized by FBI agents and just hours before she was scheduled to talk to a federal prosecutor about her involvement in the Rivera-Sternad operation. She also had been jailed by Miami cops on an old suspended driver-license warrant.

“Am I worried? Yes,” Mauricio Padilla, her lawyer, said Friday. “I have not heard or seen her. This has never happened to me before.”

Enrique “Rick” Yabor, who represents Sternad, said he and his client would not comment.

Sternad initially failed to report the cash receipts or expenditures — totaling at least $47,000 — which could violate federal campaign laws concerning financial disclosures for congressional candidates. It’s also illegal to conspire to break federal laws and launder money.

Rivera, already under a separate federal criminal investigation into his personal and campaign finances, denied any association with Sternad, who often attacked candidate Joe Garcia in the Aug. 14 Democratic primary.

Garcia won that race and now faces Rivera in the general election for the Kendall-to-Key West District 26 seat.

Rivera did not return two calls to his cellphone Friday night, and his campaign aide Javier Correoso, did not return a call or email for comment.

Republican Mitt Romney’s presidential campaign has kept its distance from the congressman. Rivera is the only one of Miami’s three Republican congressmen who has been consistently absent from Romney campaign events, such as a Univision forum in Coral Gables and a Miami rally on Thursday. Until recently Rivera had been publicly stumping for Romney, even rallying listeners on Spanish radio to join him in supporting Romney.

Meanwhile, the Democratic Congressional Campaign Committee has reserved about $1.5 million worth of air time, starting in mid October, for the Miami-Fort Lauderdale media market. A good portion of that money — though not all — is likely to be spent on the Rivera-Garcia race. In contrast, the Republican committee plans to spend nothing on Rivera.

The two faced each other in a bitter 2010 race that Rivera won in a landslide.

Garcia decided to run again this election. He faced three other Democratic candidates, including Sternad. The Garcia campaign began accusing Sternad of acting as a stand-in for Rivera and noted that the fellow Democrat, an unknown political newcomer, was using a Hialeah company that Rivera had used for years, Rapid Mail & Computer Services.