After just two days of monitoring a wiretap on a Philadelphia power broker's phone, FBI agent John Roberts knew he was working a huge case.

He heard Ronald A. White, one of then-Mayor John F. Street's closest advisers, dictating to the city's treasurer which contractors should get work on a government financial deal - and which ones should be excluded.

"Find something wrong," White ordered, telling the treasurer to dump an already-approved contractor and hire one who fit White's criteria for pay-to-play politics.

Because of the efforts of Roberts' team, Treasurer Corey Kemp was convicted in 2005 and sentenced to 10 years in prison in a scandal that rocked City Hall. White died of cancer while under indictment.

After helping put dozens of Philadelphia and New Jersey politicians behind bars, Roberts has left the FBI. The job, which he dreamed of as a child, gave him great stories to tell.

"It's the only thing I wanted to do . . . since elementary school," said Roberts, 50, who retired recently after 25 years with the FBI, 20 in Philadelphia. For the last five years, he ran the corruption squad.

As a lead agent in the City Hall case, he became something of an expert on why business and government officials go bad. He has lectured in the United States and Europe on corruption, drawing from his Philadelphia experiences in catching money launderers and serving as an undercover agent.

Roberts, who has taken a job as a senior manager in the antifraud forensic area of the Deloitte accounting firm, said people usually are corrupted gradually, rather than waking up one morning and deciding to jump into criminal activities.

"It's a slippery slope," said Roberts, who maintains a sense of empathy for many of the people he saw crossing the line into illegality.

A native of Washington, Roberts was raised by parents who ran a dry-cleaning business. He attended Maryland's Frostburg State University, majoring in economics and accounting. He is a licensed certified public accountant.

At 5-foot-11 and 180 pounds, Roberts blended in easily in frequent undercover roles.

In Memphis, where he was initially stationed, Roberts joined other agents in what was dubbed Operation Star Watch, posing as a crooked airplane broker. Based out of two dummy Memphis corporations, agents focused on drug runners seeking high-performance aircraft.

A federal grand jury indicted 25 people, but many were in Colombia and were never brought to justice. Roberts said he was targeting one suspect when the man was murdered by the Medellin cartel.

He worked other flashy cases, too, such as a 2006-09 corruption probe that netted 16 convictions, including those of New Jersey mayors, assemblymen, and school board members.

In that case, the FBI set up the phony Coastal Solutions L.L.C. insurance company near Atlantic City. Agents then taped officials in Atlantic County's Pleasantville and in North Jersey cities as they took illegal cash from the undercover FBI team in exchange for favorable treatment so that Coastal Solutions could get contracts.

In another investigation about the same time, Roberts help convict a lawyer and a regional president of Philadelphia's Nova Savings Bank in a money-laundering case. A third suspect died in an apparent suicide.

Roberts repeatedly had to confront suspects and tell them about the FBI's evidence against them. For most suspects, he said, that was "the absolute lowest point in their lives," so he had to proceed with great care.

Roberts may be best remembered for the changes his team brought about in Philadelphia politics.

After the White wiretapping, Roberts said, he and another agent played incriminating recordings for Kemp when they confronted him in a parking garage, trying to persuade him to cooperate with the investigation. Kemp refused, and eventually he received the stiffest sentence of about two dozen City Hall defendants.

Kemp's lawyer, L. George Parry, said prosecutors had overwhelming evidence against his client. Although he lost the case, Parry applauded Roberts' work.

"John Roberts is a very fair and accomplished agent," Parry said. "It's a loss for all of us that he is retiring."

Committee of Seventy vice president Ellen Mattleman Kaplan said the City Hall investigation helped bring about needed improvements in government.

"There are many more ethics protections around the city than there ever had been, and that's good," Kaplan said. Mayor Nutter has wanted to make ethics in government a priority.

Roberts' work also helped change the Police Department.

After years of sending corrupt police officers to prison, Roberts, as a supervisor, knew there was a better way to handle allegations of graft.

In 2010, he and Commissioner Charles H. Ramsey agreed to have four Internal Affairs police officers work directly with the FBI's police-corruption task force.

Each week, Roberts said, one officer reviews Internal Affairs complaints to find those that merit FBI scrutiny.

Chief Inspector Alice Mulvey, who runs Internal Affairs, said working closely with the FBI "makes everyone more efficient."

She said that as her FBI contact for the new program, Roberts "was very determined to make this work."

In Roberts' last few years as a manager, there were frosty relations with the city Inspector General's Office.

"While we had the support of the U.S. Attorney's Office, it became clear to me that John had a different concept of how other agencies should work with the FBI," Inspector General Amy Kurland said.

Last year, Philadelphia FBI chief George C. Venizelos intervened and formed a separate squad without Roberts' involvement to work police corruption cases and cases handled by Kurland's office involving government wrongdoing.

"Today . . . relations couldn't be better because we have a real partnership between investigative agencies," Kurland said.

Venizelos said those types of conflicts "happen everywhere."

He praised Kurland and Roberts, and stressed that "the work is always more important than the people."

Roberts said conflicts always happen: "Inevitably, in highly charged environments, people are going to have differences of opinion."

He gets strong applause from a wide range of people who worked closely with him over the years.

U.S. Magistrate Judge Timothy Rice, a former chief of U.S. Attorney's Office criminal unit who handled cases with Roberts, called him "a model FBI agent."

Public Defender Bradley S. Bridge said Roberts had a "sense of vision," an understanding that police, defense attorneys, and prosecutors can work together.

"There are shared interests that allow a working relationship to exist, if you're open to it," said Bridge, who sometimes arranged for his clients to give the FBI information about corrupt police. "I will miss him."

Lawyer Michael A. Schwartz, a former chief of the U.S. attorney's anticorruption unit who was a prosecutor in the City Hall case, said it was too early to use the past tense in talking about Roberts given his new job with Deloitte.

"This is no retirement job. It's a second career," said Schwartz, who now specializes in white-collar defense work. "I can't wait to work with John."