Editor's note:This story was updated at 1 p.m. Aug. 9, 2006, with new information. The Agriculture Department announced July 13 that personal identity information was not downloaded or transferred outside its computer system when it was illegally accessed in June. Because of this, the agency is no longer offering free credit monitoring services.

A hacker broke into an Agriculture Department information technology system that contained names, Social Security numbers and other employee data during the first weekend in June, department officials said last week. The security breach put 26,000 current and retired employees and contractors in the Washington, D.C., area at risk of identity theft and other cybercrimes.

Security analysts asked to comment on the incident said the federal government has become a growing target for economic crime and espionage. Despite laws that require the safeguarding of government information and personal data, the recent publicized breaches are simply the tip of the iceberg, the analysts said.

Computer forensic examiners confirmed that an unauthorized person accessed a computer system in the USDA’s Office of Operations. Although examiners could not determine whether a database containing personal information was viewed or downloaded, their forensic analysis indicated the data could be at risk, USDA officials said.

The USDA is sending letters to all affected employees and contractors and will offer them free credit-monitoring services for a year. The USDA notified them of the problem June 21 via e-mail.

The USDA incident is the latest in a string of revelations about agency security breaches, and it does not surprise Paul Kurtz, executive director of the Cyber Security Industry Alliance. “It seems like every week it’s a new federal agency,” he said. In recent weeks, the Department of Veterans Affairs and the Energy Department have also revealed details of information security incidents.

The federal government is a tempting target for economic crime and espionage, said Alan Paller, director of research at the SANS Institute. He estimates that 10 to 20 computer breaches occur weekly in which unauthorized outsiders take control of federal computer systems. Most agencies don’t know how damaging the attacks are and don’t admit they occurred, he said.

“It would be like having a thief in your house for four hours and having no videotape [so] you don’t know what he did,” Paller said. “By keeping it a secret, they take away any pressure on executives to fix it.”

The recently publicized breaches are likely just a small fraction of the real damage, with hundreds of others that could be lurking outside public knowledge, said Bruce Brody, vice president for information security at Input, a market research and consulting firm. Brody has served as chief information security officer at DOE and the VA.

“We know the controls aren’t in place that [could] prevent other [attacks] from occurring,” Brody said.

Michael Arnone contributed to this article.