At no point, she said, did she notice anything suspicious, and she suggested that she had little to help investigators, because the gunman had shot Mr. Nemtsov in the back as the couple walked side by side.

Image Anna Duritskaya, a model and the girlfriend of Boris Nemtsov, witnessed his death. Credit... Daria Buznikova/Agence France-Presse — Getty Images

Mr. Nemtsov, 55, a longstanding proponent of democratic reforms who had been in and out of jail, crumpled on the sidewalk. By the time she looked back, she said, she saw neither the gunman’s face, nor the license plate of the getaway car. She ran to a nearby snowplow, she said, to ask for help, then returned to the body and waited, by her account, about 10 minutes for the police.

The relationship of the older, famous politician and the young model from a country many Russians now believe to be their enemy has drawn intense interest from Russia’s tabloid press.

The Investigative Committee, the law enforcement organization with the lead on the case, has said it was examining a variety of possible motives, including that fellow members of the opposition killed him to create a martyr and that Islamist terrorists might be to blame, or that conflicts in his personal life might be the root.

More than a few Russian commentators and politicians floated the idea of jealousy. Vladimir Zhirinovsky, the Russian nationalist politician, for example, said of Mr. Nemtsov’s romantic life, “This young girl was not the first.” Mr. Zhirinvosky said his former colleague in Parliament “was always changing his partners, changing wives.”

Ms. Duritskaya, in the interview, denied that either of them had received threats from jealous former lovers.