Officials say there was a sudden surge in the completion of census forms in parts of the city where head counters had met resistance or found empty houses.
Chicago had been lagging behind in the census count earlier this summer until a publicity blitz boosted the completion rate. And several current and former census employees told the Chicago Tribune they were told to finish the count in a hurry and ignore the rules.
Many of the forms filled out during June showed census takers relied on neighbors to provide information about occupants of houses and apartments, rather than talk to the occupants themselves, according to the newspaper.
"We have some doubt about the quality" of the count there, said Steve Jost, a spokesman for the Census Bureau.
Census officials say they don't know if the questionable practices raised or lowered Chicago's population count. Those numbers will be released next year.
They also said they don't yet know what rules, if any, were skirted, but hope to answer that question during the recount, which will be completed in August.
"This is the kind of problem we know how to solve," Jost said. "We planned for this, that there are going to be some people out there who don't want to follow directions or may not have been trained properly."
The recount covers 9,000 households in the wealthy neighborhoods of Lincoln Park and Lake View, as well as the West Side.
City officials blamed the problem in part on doormen in affluent areas who refused to let census takers into apartment buildings.
The Chicago recount is one of a handful ordered by the census bureau. The largest recount is taking place in Florida, where suspected corner-cutting has led census officials to retrace information gathered from 71,000 households.
Despite the problems, Rep. Carolyn Maloney, D-N.Y., the ranking Democrat on the House subcommittee on the census, said she believes the count overall has been efficient and thorough.