Mayor Martin J. Walsh lashed out at former Grand Prix of Boston CEO John Casey yesterday, accusing him of lying about cooperating with federal prosecutors targeting the failed IndyCar race as part of an ongoing probe of City Hall corruption.

Walsh, responding to a report in the Herald that Casey was talking with U.S. Attorney Carmen M. Ortiz’s office and the U.S. Labor Department, said he has not been contacted by federal authorities and questioned Casey’s credibility.

“Absolutely not and I don’t think John Casey is telling the truth,” Walsh said.

Walsh’s office last night issued another statement, saying, “Mr. Casey’s dishonesty has been proven” and rebuking the Herald for reporting the former promoter’s claims.

Casey told the Herald in an interview on Monday that he has met several times with prosecutors in the U.S. Attorney’s office, the FBI and Labor Department investigators since the race collapsed in late April.

The Herald confirmed independently with other sources that the U.S. Attorney’s office and Labor Department are investigating the IndyCar debacle and have met with Casey.

“They asked for my cooperation along the way and have made great strides,” Casey said in an interview.

A Walsh spokesman said it was “disappointing the Herald would take John Casey as a trusted source.”

The investigation appears to be focused on whether the Walsh administration or other elected officials exerted pressure or withheld permits for the Grand Prix in exchange for union jobs on construction ?contracts.

The probe also could expand to other aspects of the bungled race, including whether city officials and other elected officials made demands for the local race promoters to give contracts to favored firms or charities.

Asked if he had ever talked to Casey about union pressure for Grand Prix contracts, Walsh said, “No, no.”

A Walsh spokeswoman said the city has not received any subpoenas in connection with the IndyCar race and no city officials have been called in for questioning.

Two high-ranking Walsh aides have already been indicted by Ortiz’s office on extortion charges in an alleged conspiracy to force a local music festival, Boston Calling, to hire union stagehands.

Casey canceled the IndyCar event in late April, after a city conservation commission ruled the Grand Prix had to get environmental approval to use parts of the course that ran through new flood zones.

Casey charged the city was too difficult to work with, but the Herald later reported that the Grand Prix was broke when the race was canceled, and couldn’t even give refunds to thousands of ticket holders.

Casey has subsequently been sued by several vendors, sponsors and investors looking to get their money back, and Attorney General Maura Healey also filed a lawsuit to try to get Casey to pay back ticket holders.

The Herald also reported that a Colorado judge this month ruled Casey committed fraud on a Colorado leasing company by pocketing $800,000 he claimed he was using to buy equipment.