The investigation into D.C. Mayor Vincent Gray's 2010 campaign will stretch into 2013 after prosecutors told a federal judge Wednesday that their probe is ongoing and that a key figure's cooperation is still needed.

During a status hearing for Thomas Gore, the assistant treasurer of Gray's campaign, Assistant U.S. Attorney Ellen Chubin Epstein said authorities might still call upon Gore for more help.

"We need more time for both parties to get the maximum benefit from Mr. Gore's cooperation agreement," said Epstein, referring to a provision in Gore's plea deal that calls for his assistance.

Asked by U.S. District Judge Colleen Kollar-Kotelly whether future cooperation would include private interviews or trial testimony, Epstein replied that Gore might be asked to offer "some of both."

But Frederick Cooke, Gore's lawyer, told Kollar-Kotelly he thought his client had fulfilled the promise he made to federal investigators when he pleaded guilty in May.

"He has provided the cooperation that he can provide," Cooke said. "He is very much desirous to have this matter come to a closure."

Kollar-Kotelly agreed to delay scheduling a sentencing date "at least one more time," but she said she had offered an "admonition" to prosecutors about the speed with which they should proceed.

Cooke, who said his client has not testified before a grand jury, was exasperated by Kollar-Kotelly's decision to set another status hearing for March 28.

"I'm frustrated," he said. "I just want it to be over."

Gore was the first person publicly charged in the long-running investigation of Gray's 2010 campaign, a review that dates back to March 2011.

The probe began after Sulaimon Brown, a minor mayoral candidate, went public with allegations that the Gray campaign promised him cash and a government job in exchange for his agreement to remain in the contest and criticize Adrian Fenty, the incumbent and Gray's top political rival.

More than a year into the investigation, Gore pleaded guilty to four charges: a federal count of obstruction of justice and three District counts of helping to make campaign contributions in the name of others.

The obstruction charge, prosecutors said, stemmed from Gore's decision to shred a notebook containing a record of payments to Brown around the time the ex-candidate went public with his allegations.

Authorities have filed charges against two other people associated with the campaign.

Earlier this year, Kollar-Kotelly sentenced a former consultant, Howard Brooks, to probation for lying to investigators who were looking into the payments to Brown.

Jeanne Clarke Harris, a communications consultant, is due to appear in court for a status hearing next week. She has pleaded guilty to helping orchestrate a $653,800 "shadow campaign" designed to boost Gray's prospects.

People familiar with the probe have said that Jeffrey Thompson, a major city contractor and prolific campaign donor, bankrolled that illicit effort, though he has not been charged.

Gray has not been charged with any wrongdoing, and he has vowed to serve out his term, defying repeated calls for his resignation.