Mr. Baranov, in an interview Thursday morning, emphatically denied any criminal activity. He said he had “absolutely not” entered doped athletes into United States competitions nor had he made inappropriate payments to race organizers. He said he had been unaware of the government’s investigation.
A spokesman for the New York City Marathon said that organizers had no knowledge of criminal activity within their operation and that no one in the organization had received a bribe from Mr. Baranov.
“If contacted, we will cooperate fully with the authorities,” the organization said in a statement Thursday.
The investigation is part of the Justice Department’s broader inquiry into doping schemes that have already led to penalties against scores of Russian athletes and officials. The federal inquiry, which escalated as this weekend’s race approached, with plainclothes agents monitoring Mr. Baranov’s prewar apartment building between Broadway and West End Avenue, includes the Federal Bureau of Investigation and the United States attorney’s office for the Eastern District of New York in Brooklyn.
The authorities are exploring possible racketeering and money-laundering offenses dating back several years and involving the organizers of marquee American track and field competitions.
The New York Times reported in May that the Justice Department had opened an investigation into state-sponsored doping by dozens of Russia’s top athletes, intending to scrutinize anyone who might have facilitated unclean competition in the United States or who might have used the United States banking system to enable a doping scheme. Grigory Rodchenkov, the former longtime head of Russia’s antidoping laboratory, had told The Times that the Russian government ran an elaborate program to help the country’s athletes use banned, performance-enhancing drugs and go undetected.
More than 100 Russian athletes were barred from the 2016 Summer Olympics in Rio de Janeiro after the revelations, and employees of the Russian government were refused accreditation for the Games. Participants in earlier Olympics, too, have been affected, with about three dozen Eastern Europeans having been disciplined in recent months after retests of their urine samples from the 2008 and 2012 Summer Games.
Now, United States law enforcement officials have focused on the agents behind those athletes, and the money they may have routed through American banks — an approach that echoes the continuing criminal inquiry that has rocked FIFA, soccer’s world governing body. In the FIFA case, the same federal prosecutors’ office in Brooklyn brought charges of bribery, racketeering, money laundering and wire fraud against some of soccer’s top officials. Eighteen people have been convicted.
The authorities are believed to be looking for similar offenses in their inquiry into Mr. Baranov and distance running, once again elevating a sports scandal into a possible criminal case of international interest. They are investigating whether race officials accepted bribes to allow athletes using banned substances to compete in their events, and whether people knowingly entered drugged athletes into competition, possibly defrauding organized running.
If payments — for appearance fees, race winnings or illicit reasons — traveled through American banks, the government could claim jurisdiction. Prize money is often awarded to the top 10 or 15 finishers.
The Russian doping scandal has captured the attention of the American authorities on multiple fronts. They have also investigated computer hacks of the World Anti-Doping Agency and the United States Anti-Doping Agency; both breaches were attributed to a Russian cyberespionage group believed to be associated with the G.R.U., the Russian military intelligence agency.
Dr. Rodchenkov, who fled to the United States last year with his files on the sports ministry’s schemes, is facing scrutiny from the same federal prosecutors who are pursuing Mr. Baranov. He has provided investigators with information.
Some of Mr. Baranov’s clients, Russian distance runners who trained under Russian national team coaches, could have been among those who received Dr. Rodchenkov’s signature cocktail of liquor and three anabolic steroids, a combination he said helped speed up absorption of the drugs and limit the time during which they could be detected.
Mr. Baranov, 50, is also no stranger to sports science. He graduated from the Lesgaft National State University of Physical Education, Sport and Health in St. Petersburg, Russia. Records show he settled in New York as early as the 1990s. He has run the New York City Marathon six times, finishing in 2 hours 36 minutes 39 seconds in 2000, his best performance.
In 2003, he founded his company, Spartanik Running School, “to bring the dominance of Eastern European athletes at the Olympic competitions of 1980s back to the highest level,” according to Spartanik promotional materials.
Over the past decade, he has represented dozens of notable runners, predominantly from Russia, Ukraine, Belarus and Kyrgyzstan.
Some of them, like Lyubov Denisova — who has repeatedly been among the top-three finishers in the New York City, Boston and Los Angeles Marathons — have failed drug tests. Ms. Denisova was disciplined for elevated testosterone levels in 2007, months after she set a record, 2:27:19, at the 2006 Honolulu Marathon.
“She was 100 percent sure she was clean,” Mr. Baranov told The Honolulu Star-Bulletin at the time. “It was a huge surprise to everyone.”
Another of Mr. Baranov’s high-profile clients — Liliya Shobukhova, who won the Chicago Marathon in 2009, 2010 and 2011 — was exposed by Mr. Baranov.
Mr. Baranov told sports ethics investigators in 2014 that Ms. Shobukhova had been subjected to extortion and had paid half a million dollars to top track and field officials to conceal her doping violations, enabling her to compete in the 2012 Olympics. (She was subsequently stripped of several titles.)
As a result, Mr. Baranov has been hailed as a whistle-blower in some news reports, credited with providing some of the earliest testimony about Russian sports doping and associated allegations of bribery and corruption.
Still, the track and field officials he spoke out against — men who this year were barred from the sport — insisted that Mr. Baranov should not be overlooked, according to an ethics decision published this year by the sport’s global governing body. Mr. Baranov’s “disgusting reputation in the world of athletics is well known,” said Alexei Melnikov, the former head coach of Russia’s distance runners and racewalkers.
Since at least last year, New York City Marathon officials have grown skeptical of Mr. Baranov and shunned his athletes. He lamented in The Times in November 2015 that his athletes were being unfairly punished for Russia’s doping scandal.
“I have a feeling why,” Mr. Baranov said of the exclusion of his runners from the race. “It’s because of the doping problem in Russia. I know there’s a doping problem there. But just because there’s one bad apple doesn’t mean in Russia there are all bad apples. And in other Eastern European countries, the apples are even from completely different trees.”
The New York City Marathon spokesman said that no Russian athletes were registered to run on Sunday, in keeping with guidelines from global officials who have barred Russian athletes from competition.
On Thursday, Mr. Baranov said that none of his clients were registered to run in Sunday’s marathon. “It’s a private company,” he said of the organization that runs the race. “They can choose what they want to do.”