In the summer of 2011, the arrest of U.S. Rep. David Rivera seemed all but certain.

Agents with the Florida Department of Law Enforcement had waded through piles of credit-card receipts and banking records, tracing thousands of dollars from Rivera’s political campaigns to his personal accounts. Miami-Dade prosecutors were preparing a “draft” complaint charging the Republican congressman with 52 counts of theft, money laundering and racketeering.

The lengthy probe of Rivera’s finances “unequivocally explains the theft and/or fraud of campaign funds,” FDLE inspector Brett Lycett wrote in a July 5, 2011, e-mail to a prosecutor. “We believe the violations are quite evident.”

But in the ensuing months — after Rivera’s lawyer poked holes in the case — the investigators’ confidence gave way to prosecutors’ increasing skepticism about the potential charges. The 52-count complaint accusing Rivera of systematic misspending of campaign funds was never filed; instead, prosecutors would write a 16-page memo explaining why they believed they could not arrest Rivera for anything.

Newly released e-mails and other records from the Rivera investigation show the increasing tension between the FDLE and the Miami-Dade state attorney’s office as the case dragged on through this past April, when prosecutors — under pressure from the FDLE to make a decision — finally dropped the high-profile case.

Some six months earlier, in November 2011, FDLE Commissioner Gerald Bailey urged Miami-Dade State Attorney Katherine Fernández Rundle to move more quickly, worried that the time lag would prevent them from bringing some charges against Rivera.

“While I appreciate the need for careful deliberation in matters such as this, I am concerned that the statute of limitations may become an issue if a decision regarding prosecution is further delayed,” Bailey wrote in a Nov. 3 letter to Rundle.

By then — unbeknownst to Bailey or his investigators — Rundle’s prosecutors had already offered to drop the criminal charges against Rivera if the congressman agreed to admit to civil election-law violations, e-mail records show.

FDLE agents grew so frustrated with the delays that they approached the state attorney in Leon County, Willie Meggs, to see if he would be willing to pursue the case, according to an internal FDLE memo from January.

Meggs told The Miami Herald he was contacted “informally” by an FDLE agent, but he did not discuss details of the Rivera case, out of deference to Rundle. “I didn’t want to overstep my bounds,” Meggs said.

Rivera, a Miami Republican who is now seeking re-election, has long denied any wrongdoing. In a statement issued through his campaign, he criticized the investigation as “frivolous.”

“The drafting of the complaint was an obvious attempt by investigators to intimidate, pressure and extort Congressman Rivera into a negotiated settlement,” Rivera said.

Rivera remains the target of a separate federal investigation by the FBI and the IRS.

Even after Rivera’s attorney, Michael Band, presented documents to prosecutors and investigators that he said showed that Rivera did not misspend any campaign money, the FDLE agents remained convinced that Rivera should be charged, the records show.