An FBI probe into corruption in Allentown City Hall struck at the heart of the mayor's inner circle Monday, as prosecutors filed charges against the city's one-time finance director.

Garret Strathearn, who resigned in May, is the highest ranking city official to be charged in an investigation that has already wrung guilty pleas from a prominent developer and a former city lawyer, and appears to lead to Mayor Ed Pawlowski's office.

Identified in court filings only as "Public Official #3," Pawlowski has not been charged. He did not return calls for comment Monday.

In filings in the case, including the charging documents related to Strathearn, prosecutors portray Pawlowski, who aspired to run for a U.S. Senate seat, as the chief architect and beneficiary of the pay-to-play scheme.

They suggest he instructed his political operatives to seek campaign donations from businesses that had profited from their relationship with Allentown, and that city officials "give preferential treatment to certain of his past and potential political donors."

The charges filed against Strathearn, 68, of Sea Girt, N.J., center on his 2014 role in shepherding a contract for Allentown's delinquent tax collections to a Montgomery County law firm that Pawlowski allegedly hoped would donate generously to his campaign fund.

Northeast Revenue, a Wilkes-Barre collection service, and Jenkintown law firm Friedman Schuman were awarded the contract in February 2014 after teaming up to submit a bid. Principals from both companies donated $15,000 to Pawlowski's campaign between 2013 and 2014.

When the city's contract for delinquent tax collections came up for renewal in August 2013, Pawlowski sought a change.

Though one law firm - identified in court filings as "Law Firm #1" - had performed the job for the city for years, the mayor, according to court filings in Strathearn's case, was not happy with the financial support shown by the firm in his past bids for political office.

He allegedly instructed Strathearn and others to rig the bidding process in favor of the Northeast Revenue-Friedman Schuman partnership because he believed its managers would be more generous.

At one point, prosecutors said, Strathearn made up conversations he had with references provided by all three companies bidding on the contract in order to make the Friedman Schuman partnership look better and diminish its competitors before the committee.

Strathearn, Pawlowski, and an assistant city solicitor, Dale Wiles, who has also pleaded guilty in the case, all later lied to the FBI about their involvement in the delinquent tax collection contract, prosecutors said.

FBI agents raided City Hall in July, seeking information on more than two dozen people and businesses that do work for the city.

Strathearn faces one count of conspiracy to commit mail and wire fraud. He was charged by way of a criminal information - a sign that typically suggests a defendant intends to plead guilty. He did not immediately respond to requests for comment Monday.