A travel reservations Web site used by several federal agencies was hacked last week, and it shunted unsuspecting users to a malicious domain, according to information that Computerworld has obtained.

The site, GovTrip.com, is currently unavailable to federal employees through their offices' intranet; the version accessible via the public Internet is also offline.

Sometime before Feb. 11, hackers breached the site, then modified it to redirect users to a rogue URL that in turn directed attack code against their systems, according to the General Services Administration (GSA) and e-mail sent to federal workers that Computerworld has seen.

"Last week, some users of GovTrip, when logging on to the GovTrip site, were redirected to a site that delivered malicious software to their computers," an e-mail sent to employees of the U.S. Environmental Protection Agency (EPA) read. "The incident, which also affected other federal agencies, was quickly identified by systems security. At this time, the GovTrip site is not safe to use and should not be accessed."

Today, GSA spokesman Robert Lesino confirmed the GovTrip hack and said the redirect hit users on Feb. 11. "The incident was quickly identified," said Lesino, who declined to answer specific questions, citing the ongoing investigation. He also said that no user information was believed to have been compromised by the hack.

GovTrip is used by several U.S. government agencies, including the EPA and the departments of Energy, Health and Human Services, the Interior, Transportation, and the Treasury, to make travel reservations, as well as to reimburse workers for travel expenses. The site is operated by defense contractor Northrop Grumman Corp. from an office in Northern Virginia.

As of midday Wednesday, GovTrip remained offline. "GovTrip.com is currently unavailable," a message on the site read. "All travelers who need to make travel reservations should contact their Agency Travel Management Centers."

The message to EPA employees said that the agency did not know when the site would be brought back online.

Also unknown is the malware that tried to infect workers' PCs when they accessed GovTrip last week. Lesino said that the hack had been reported to the U.S. Computer Emergency Response Team (US-CERT), as is standard practice. US-CERT, however, does not publicly disclose the details of the incident reports it receives from federal agencies.

This is not the first time a site or server related to the government has been breached. Last week, for instance, the Federal Aviation Administration acknowledged that hackers had broken into its network and may have stolen medical records of some 45,000 current and former employees.

"The GSA, the vendor and customer-agency IT specialists are moving swiftly to identify short-term and long-term measures to find the source of the incident and to prevent such an incident from recurring," Lesino said.