Prosecutions of swindlers, crooked investment advisers and other white collar criminals have fallen to a 20-year low nationwide, a downward trend evident in Minnesota despite notorious scams in recent years.

That doesn't mean that fewer frauds are underway, however.

U.S. Attorney Andrew Luger says white collar crimes are becoming more complex in the Internet age, while federal resources for investigating them are strained. So he's turned for help to federal agencies that haven't traditionally investigated white collar crimes and state and local law enforcement.

That kind of cooperation is what led to the indictment of Dr. Elena Lev Polukhin of Minnetonka, who appeared in federal court Wednesday to answer charges that she was a central player in a conspiracy to write prescriptions for pain medications in exchange for more than $40,000 in kickbacks.

Luger, a former white collar prosecutor and defense attorney, announced the indictment of Polukhin, together with related charges against two of her alleged co-conspirators, warning that federal agencies in Minnesota remain on the lookout for fraudsters.

Minnesota got a reputation for white collar crooks in the wake of the recession with the prosecution of Wayzata businessman Tom Petters, whose $3.65 billion Ponzi scheme was one of the largest in U.S. history. More recently, federal prosecutors in Minnesota have charged a large ring of thieves who specialized in stealing smartphones; several organized crime operations that specialized in identity fraud and related crimes; and 14 investment fraud schemes in 2014 alone.

Still, the number of prosecutions is on a downward trend in Minnesota.

Luger noted that white collar crime prosecutions tend to follow a boom-bust pattern, partly because complex, multi-jurisdictional schemes occupy investigators and prosecutors for months or even longer periods, constraining the new cases they can pursue.

"Look, we prosecuted 167 white collar defendants last year, which is … more than twice what was prosecuted the year before I got here," Luger said. After that spike, he said, it's not surprising that this year appears headed for a decline.

Federal agents in the Twin Cities who work white collar cases have complained privately about declining personnel in recent years, constraining the number of cases they can investigate.

Historically, most white collar cases in the Twin Cities have been fielded by the FBI, the Internal Revenue Service (IRS) and the U.S. Postal Inspection Service, Luger said.

The FBI does not release information about the number of agents it has in a particular office. Kyle Loven, a spokesman for the FBI's Twin Cities office, said that while the number of agents focusing on terrorism cases in the office has increased — the office is about to add a fourth counterterrorism squad — the agency nonetheless has maintained two full squads of white collar investigators. He said the number of agents working white collar cases has remained "relatively steadfast."

Like the FBI, the IRS declined to say how many criminal investigators work in the Twin Cities, but they are likely declining. The number of IRS agents nationwide has declined 32 percent since 1995, and is down 16 percent since 2011 alone, according to the agency's 2014 annual report.

The U.S. Postal Inspection Service said it has cut its agents in the Twin Cities office by 20 percent since 2010.

Luger said it's true that the number of federal agents working white collar cases is down in Minnesota and across the country. "That's the new reality that we have to grapple with," he said. "And one of the solutions that we have come up with here is to increase the nontraditional white collar investigators."

Luger cited several major cases coordinated between federal agencies and the Minnesota Bureau of Criminal Apprehension (BCA), the Minnesota Department of Commerce fraud division, the multi-jurisdictional Minnesota Financial Crimes Task Force, and local police and sheriff agencies.

"The Secret Service has stepped up tremendously and is a significant part of the white collar world we occupy now," Luger said.

Lou Stephens, special agent in charge of the Secret Service in Minnesota and the Dakotas, said the number of agents in his office has remained flat in the four years he's headed the office. Even so, he said, it's been "a constant struggle" to get enough investigators. That's because Secret Service staffing nationwide has declined, Stephens said, so his agents get pulled for special assignments, such Pope Francis' visit to the United States.

Stephens said the Secret Service has ramped up its white collar investigations by building partnerships with state and local investigators.

Mike Rothman, who was appointed Minnesota Commerce commissioner in 2011, reorganized the department's 10-year-old fraud bureau to take a more systematic approach, said spokesman Ross Corson. Although the fraud bureau's staffing remained flat, he said, turnover resulted in the hiring of more experienced investigators who tackled more complex cases.

The Minnesota Financial Crimes Task Force, which is made up of federal, state and local law enforcement, also has kicked up its game. In 2014, its investigations led to 56 federal indictments, up from just 9 the year before.

Luger said no one should doubt that white collar crimes — especially investment fraud — remain a top priority of his office. So much so, that last year he personally took the guilty plea of Mark Holt, 45, a former securities broker, investment adviser, and now disbarred lawyer who stole more than $4 million from his clients.

"The courtroom was packed with the victims, and most of them were elderly," Luger said. "They told me their stories, and it's heartbreaking. … The injustice that is done to a retiree who has done everything right his or her whole life, because of someone's greed, is overwhelming. And I always have taken it very, very seriously."