WASHINGTON, June 20 — Federal environmental officials misled Lower Manhattan residents about the extent of contamination in their condominiums and apartments after the collapse of the World Trade Center, according to a preliminary report released on Wednesday by the Government Accountability Office.

According to the report, made public during a Senate subcommittee hearing, the Environmental Protection Agency did not accurately report the results of a residential cleanup program in 2002 and 2003. More than 4,000 apartments in Lower Manhattan were professionally decontaminated in that program, and the agency reported that only a “very small” number of air samples taken in those residences showed unsafe levels of asbestos.

But the agency failed to explain that 80 percent of the air samples were taken after the apartments had already been cleaned.

“That was misleading,” said John B. Stephenson, director of the natural resources and environment division of the Government Accountability Office, an investigative arm of Congress. He spoke after testifying at a hearing of the Subcommittee on Superfund and Environmental Health of the Senate Committee on Environment and Public Works, which is reviewing the government’s response to environmental and health issues at ground zero.