A former aide to New Jersey Gov. Chris Christie walked back a previous claim that the governor lied about his staff’s involvement in the 2013 lane closures at the George Washington Bridge.

Christina Renna, the former director of Mr. Christie’s now-defunct Office of Intergovernmental Affairs, testified in federal court in Newark that she had made a “poor choice of words” in a 2013 text-message exchange with another Christie associate. In that exchange, Ms. Renna said Mr. Christie “flat-out lied” in a news conference in which he denied his re-election campaign or administration were involved in or had knowledge of the lane closures

“It was a poor choice of words,” Ms. Renna said. “I had no knowledge of whether the governor was lying or not during that press conference.”

“I knew personally what Governor Christie was saying seemed to contradict what I was told,” she added.

Ms. Renna said Thursday she was reacting to what she had learned in a succession of phone calls the previous evening, Dec. 12, with Bridget Kelly, then Mr. Christie’s deputy chief of staff.

Ms. Kelly and Bill Baroni, formerly a top executive at the Port Authority of New York and New Jersey, face charges for their alleged roles in the lane closures. Prosecutors say they helped orchestrate a scheme to create traffic jams in Fort Lee, N.J., to punish the borough’s Democratic mayor for not endorsing Mr. Christie, a Republican. Both have pleaded not guilty. Another former Port Authority official, David Wildstein, has pleaded guilty to masterminding the scheme and is cooperating with prosecutors.

Also Wednesday, prosecutor Lee Cortes asked Ms. Renna about the Christie administration’s efforts to reward and punish local officials from whom it sought endorsements.

Ms. Renna described the inner workings of the intergovernmental affairs office, a unit in Mr. Christie’s administration once run by Ms. Kelly that focused on the needs of municipalities and local officials. In practice, Ms. Renna and other witnesses testified, the office provided a clearinghouse of information of politicians who did or didn’t support Mr. Christie’s re-election in 2013. It worked closely with his campaign to negotiate political endorsements, and to freeze out those who wouldn’t back the governor.

For example, prosecutors displayed a June 2013 memo from Ms. Renna to Ms. Kelly and Bill Stepien, who had previously held Ms. Kelly’s job before leaving to manage Mr. Christie’s re-election campaign. The memo sorted mayors into several groups, separating those who had already endorsed Mr. Christie from others who needed more persuading.

Those who had agreed to endorse Mr. Christie were more likely to receive friendly treatment, Ms. Renna testified. That treatment included “Mayor’s Days,” in which state officials such as cabinet members would be dispatched to meet with local officials who had special projects or needs in their towns.

It was the deputy chief of staff—first Mr. Stepien and later Ms. Kelly—who could also designate some mayors “hands-off,” she testified, meaning the office would make “no proactive contact” with officials who were “for whatever reason not in favor with the administration.”

Ms. Renna testified that she only discovered that Mark Sokolich, the mayor of Fort Lee and ostensible target of the lane-closure scheme, was in that category when Ms. Kelly reacted angrily upon learning that a staffer had reached out to the mayor for a meeting. That meeting was in mid-August 2013 when prosecutors say Mr. Wildstein was already conspiring with Ms. Kelly and Mr. Baroni to flood Fort Lee with traffic.

Ms. Renna described an increasingly nervous series of phone calls she exchanged with Ms. Kelly on December 12, 2013, in which Ms. Kelly told her Mr. Baroni would be forced to step down from the Port Authority. Ms. Kelly said she was anxious because of inquiries about her knowledge of the lane closures from Kevin O’Dowd, then Mr. Christie’s chief of staff. Ms. Kelly also asked Ms. Renna to delete an email she had sent her during the closures in which she responded to a report that Mr. Sokolich was furious about the closures with a single word: “Good.”

Ms. Renna testified that at one point Ms. Kelly made a clear reference to the lane closures: “You know Christina, if someone tells me something is ok, who am I to question them,” Ms. Renna said.

When Ms. Renna tried to reassure Ms. Kelly that it was understandable to follow a direction from Mr. Stepien or Mr. Wildstein, she said, “Bridget cut me off and said, ‘I don’t need your vindication, Christina.’”

Ms. Renna said she deleted the message the following day, after sending a copy to another personal email address for safe-keeping. That morning, Mr. Christie held a press conference in which he insisted none of his senior staffers were involved in the lane closures. He also announced Mr. Baroni’s departure from the Port Authority while praising his service.

Ms. Renna said that had initially prompted her to text-message a confidant and fellow Christie associate, Peter Sheridan, to warn that Mr. Christie had “lied,” and that if emails from his administration or campaign were unearthed via a subpoena or legal discovery “it could be bad.”

On Thursday, she moderated that view: “I knew personally what Governor Christie was saying seemed to contradict what I was just told,” Ms. Renna testified. “I actually had no knowledge of whether Governor Christie was lying or not.”