The agency charged with prosecuting state public corruption cases wrapped up an investigation into state Sen. Wendy Davis last year without finding any issues worth pursuing, its director said, and did not uncover anything it believed it should refer to the FBI.

The Public Integrity Unit closed an investigation into a complaint made against Davis’ law firm last year. The Dallas Morning News reported Friday that documents related to Davis’ legal work as a lawyer for the North Texas Tollway Authority are part of an FBI inquiry into the agency's board members, citing a letter from the Public Integrity Unit about its own closed investigation into Davis.

Gregg Cox, the Public Integrity Unit's director, said his agency obtained documents from the FBI, not the other way around, when the Public Integrity Unit launched an investigation into Davis in 2012. The Public Integrity Unit is run by the Travis County District Attorney's office, which is led by Democrat Rosemary Lehmberg. It is tasked with prosecuting public corruption cases statewide. Its investigation into Davis was prompted by a complaint filed against her by then-state Rep. Mark Shelton, who was running to unseat Davis in the Senate.

Shelton’s complaint alleged that Davis had allowed her firm’s work with the NTTA to influence her votes on bills that impacted the agency. Davis has long said she never let her legal work or clients influence her public service. Her campaign said Friday that she had no reason to believe she was the target of any investigation.

Travis County prosecutors had seen news reports about an FBI investigation into NTTA board members and asked the federal agency for any relevant records.

“The FBI was looking into the NTTA so we reached out to them and, you know, law enforcement to law enforcement, they shared stuff," Cox said.

Cox said his unit did not find anything worth pursuing in its investigation into Davis and that his unit did not provide any documents to the FBI.

“We shut it down before it ever went to the grand jury,” Cox said. “It became obvious that if anything happened, it all happened in the Metroplex. We didn’t have venue or jurisdiction over that so we shut it down.”

Cox said that whenever his agency finds information that could be useful for a case in another venue, his office typically refers that information to the relevant agency, such as a local district attorney’s office or federal authorities.

“I know we didn’t refer anything in this case,” Cox said. “We didn’t refer anything to anybody else.”

Davis' law firm is based in Fort Worth. Melody McDonald, spokeswoman for the Tarrant County District Attorney, which has jurisdiction over crimes committed in Fort Worth, said her office is “not pursuing or involved in any investigation” involving Davis or her law firm.

The Dallas Morning News requested files in March related to the Public Integrity Unit’s investigation of Davis, which closed in August 2013. The unit sought approval from the Attorney General's office to keep the records private — primarily because they were the product of a confidential investigation, and, in the event they needed a secondary defense, Cox said, because "we needed to protect records to an investigation by another law enforcement agency."

"We felt we had a duty to protect that information," he added.

The language was stronger in the letter the Public Integrity Unit sent to the Attorney General's office making that plea. An assistant county attorney wrote that the information was “the subject of an open investigation with the Federal Bureau of Investigation” and that release of the information “would interfere with the FBI’s prosecution of the crime underlying the information.”

It is longstanding practice for prosecutors to withhold investigative files, which may contain unproven allegations that are gathered as part of a grand jury process. In its response to the county's request to withhold the records, the Attorney General's office said the request from The Dallas Morning News did not present "a novel or complex issue" and ruled that the records could be withheld.

The FBI has never publicly acknowledged the existence of an investigation related to the NTTA. The investigation became publicly known due to NTTA documents and comments from NTTA officials. Reached Friday, a spokeswoman for the FBI’s Dallas office declined to comment.

NTTA spokesman Michael Rey said his agency is not a target of an FBI investigation but that “one or more current or former board directors were.”

“An attorney for former director David Denison said last year that he had been cleared of investigation,” Rey added. “That is the only information I have about the investigation.”

Just as they did in 2012, the details of Davis’ legal work for the NTTA have emerged again in her current campaign for governor. The Greg Abbott campaign’s allegations against Davis have echoed those of Shelton’s earlier complaint, particularly the focus on two bills that Davis voted on as a senator relating to how toll agencies can handle drivers who decline to pay their tolls. Shelton alleged that Davis changed her position on the issue in order to win a collections litigation contract with the agency. Both Davis and the NTTA said at the time that there was no connection between the bill and the agency’s hiring of her law firm. Davis said that she didn't change her position but voted for the bill that offered a “better solution.”

On Friday morning, Abbott’s campaign accused Davis of hiding ethical misdeeds.

“Texans deserve to know the truth about Sen. Davis' involvement in a matter the FBI is investigating,” campaign spokesman Matt Hirsch said.

Zac Petkanas, a Davis campaign spokesman, said the campaign is not aware of being the target or subject of any investigation.

“Abbott knows this is an old political charge that was started by Wendy Davis' opponent in the 2012 campaign and that anyone who looks at this matter will conclude that Wendy Davis has always fought in the best interest of her constituents,” Petkanas said.

--By Aman Batheja and Jay Root, The Texas Tribune

This story originally appeared in The Texas Tribune.