On the heels of another guilty plea, the top federal prosecutor in Atlanta said Wednesday that an ongoing investigation into corruption at City Hall has expanded in scope and more resources are being added.

The additional staff from the U.S. attorney's office, the FBI and the IRS are needed "to bring a conclusion to the investigation expeditiously and thoroughly," U.S. Attorney Byung J. "BJay" Pak told reporters. He added that he expects to see more indictments in the case, which has already seen charges brought against six people, including several who served in high-ranking positions in the administration of former mayor Kasim Reed.

Pak declined to comment on whether he expects the investigation to lead to wrongdoing by Reed, but he said that when there are repeated instances of corruption, "you have to take a look at who sets the tone at the top." Reed, a high-profile Democrat, was first elected mayor in 2009 and was term-limited when he left office in January.

Pak encouraged anyone else who might have violated the law to come forward.

"To them, the question is not if, but when, we're coming," he said.

By accepting responsibility for their actions and cooperating with the government, Pak said, "they will get the benefit and the opportunity to substantially reduce their criminal liability and potentially even obtain immunity from prosecution."

Pak spoke after Katrina Taylor-Parks, who served as Reed's deputy chief of staff, pleaded guilty to conspiracy to commit bribery. Parks, 49, was the sixth person charged as a result of the investigation.

Parks met privately with a city vendor multiple times from late 2011 through mid-2013, and they discussed the vendor's desire for city work and how the vendor could get a sole-source contract with the city, prosecutor Jeffrey Davis said in court.

The vendor paid Parks thousands of dollars and paid for services on her behalf, and she knew the vendor wanted her to use her position and power to help him get contracts, Davis said.

For example, the vendor paid $2,000 to KTP Solutions, a company owned by Parks, in January 2013 and again in March 2013, Davis said. From January to July 2013, the vendor received five payments from the city totaling $99,999.

Parks told U.S. District Judge Steve Jones that she wasn't aware at the time that she was engaging in illegal activity, though Pak told reporters the evidence shows that she did know.

Prosecutors say she also falsely stated in annual financial disclosure statements that she wasn't self-employed and was employed only by the city and that she never received more than $5,000 in annual income from any source other than the city. Parks never told the city about her ongoing financial relationship with the vendor.

In interviews with the FBI in November of last year and February of this year, Parks falsely said she had never taken money from the vendor, prosecutors said.

Parks will be sentenced later and faces up to five years in prison. Her plea agreement says prosecutors will ask for a sentence at the lower end of the applicable sentencing guidelines. Also as part of the plea deal, Parks agreed to pay restitution and to cooperate with the government's investigation.

A federal subpoena that targeted Parks in April also sought city credit card statements and other information for Reed; his brother Tracy Reed, a former city employee; and former city director of human services and political consultant Mitzi Bickers.

Bickers was indicted in March and accused of soliciting and accepting payments to help steer lucrative city contracts to two construction contractors and their companies. She has pleaded not guilty. She also appeared in federal court Wednesday, where her lawyers asked for and received more time to review documents in the case.

Four other people have pleaded guilty and been sentenced to prison as a result of the investigation. They include the city's former chief procurement officer, two construction contractors and a man employed by Bickers who tried to intimidate one of the construction contractors to keep him from talking to federal investigators.