The mystery of the Chinese Paralympic champion in the New York City Marathon took a while to unravel.

The champion, Zou Lihong, was supposed to re-enact a great showdown from the Rio Games, where she beat the favored American, Tatyana McFadden, by a hair in the wheelchair marathon. But when McFadden crossed the Central Park finish line as the champion on Sunday, she hadn’t seen Zou for miles. None of the competitors had. Questions to a race official yielded no clue as to what had happened.

So where did Zou end her first competition in the United States?

In Brooklyn, halted by a flat tire near Mile 13, upset and among strangers who had no idea who she was. But they wanted to help.

Three of them were especially eager — a software engineer, a marketing representative and a young New York City police officer who grew up on Long Island but had never seen the marathon in person until he took up his post on Sunday.