The Census Bureau inadvertently posted personal information from 302 households on a public Internet site multiple times over a five-month period, the bureau said Wednesday.
The information included names, addresses, phone numbers, birth dates and family income ranges, said Ruth Cymber, the agency's director of communications. No Social Security numbers were posted, and there is no evidence that the data was misused, Cymber said.
But, she added, posting the information violated bureau policies and federal law.
The bureau is in the process of contacting the households, located in nine states and the District of Columbia, to offer free credit-monitoring services.
"A breach of this kind is unacceptable," Census Director Charles Louis Kincannon said in a statement. "We are strengthening our internal procedures to further safeguard our data to prevent a recurrence."
The information was on and off the public Web site from October to Feb. 15 as Census employees working from home tested new software, Cymber said. The workers were supposed to use fictitious information to test the site, but they inadvertently mingled data from the bureau's Current Population Survey, a monthly survey best known for generating the nation's employment statistics.
Cymber said the real and fictitious data were indistinguishable. The information could have been accessed through a search engine on the Census Bureau's Web site used to disseminate large data files. She said she didn't know whether the data actually was accessed by anyone.
Cymber declined to say how many employees were involved, though a release from the bureau said "appropriate administrative action" has been taken, pending the outcome of an ongoing investigation.
The bureau also referred the matter to the Commerce Department's inspector general. The Census Bureau is part of the Commerce Department.
The affected households were located in Alabama, Alaska, Arkansas, Arizona, California, Colorado, Delaware, Florida, Connecticut and Washington, D.C.
The incident comes six months after the Census Bureau acknowledged losing 672 laptop computers since 2001, including 246 that contained personal data. Most of the computers were used by workers gathering survey information in communities.