NEW YORK (CBSNewYork) — Federal law enforcement officials are reportedly investigating whether a Russian sports agent may have conspired with New York City Marathon organizers to allow athletes using banned substances to compete in the race.

The New York Times reported Thursday that federal officials have been conducting a surveillance operation on Andrey Baranov’s Upper West Side apartment and that the investigation has escalated in the days leading up to Sunday’s marathon. The probe also includes marathons beyond New York, the report said.

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In an interview with the Times, Baranov said he neither entered doped athletes in U.S. marathons nor made inappropriate payments to race organizers. He added that he was unaware of the investigation.

A New York City Marathon spokesman denied to the newspaper that event officials had received any bribes from Baranov. 

The investigation is being conducted by the FBI and the United States attorney’s office for the Eastern District of New York.

In May, the FBI launched a probe into allegations of doping by Russian Olympians after Grigory Rodchenkov, the former director of the drug testing lab at the 2014 Winter Olympics in Sochi, claimed there was a state-sponsored doping program and that his lab helped cover up positive test results.

More than 100 Russian athletes were barred from the 2016 Summer Olympics in Rio de Janeiro, and some past Russian Olympians have been stripped of medals after their urine samples were retested.

U.S. officials are now focusing their investigation on sports agents linked to doped athletes and whether illicit payments may have been routed through American banks, the Times reported.