A "sophisticated cyberattack" has been detected at Oak Ridge National Laboratory over the last several weeks, and authorities suspect the hackers are based in China.
The breach might have compromised the personal information of thousands of visitors to the lab, according to a communiqué sent to employees.
The intrusion is under active investigation by multiple agencies. FBI and Department of Homeland Security officials tell ABC News they believe the attacks originated in China with Chinese entities probing U.S. systems.
Investigators have not been able to determine whether the attacks came from government or private entities in China.
The statement, from Laboratory Director Thom Mason, said the attack "appears to be part of a coordinated attempt to gain access to computer networks at numerous laboratories and other institutions across the country."
Other federal labs, including Los Alamos National Laboratory in New Mexico and California's Lawrence Livermore National Laboratory, have also been targeted in the scheme.
Livermore lab spokesman Stephen Wampler tells ABC News that the facilities employees received "approximately 1,000 spam-type e-mails with attachments" in October and November, but said the lab's cybersecurity systems thwarted the attempted attack.
"As a result, there was no compromise of data at our laboratory," he said.
A Los Alamos spokesman said the lab notified employees on Nov. 9 that a "malicious, sophisticated hacking event" affected a small number of computers on the facility's unclassified network.
"A significant amount of data was removed," the spokesman said. "The exact nature of the information is currently under computer forensic investigation."
As for the Oak Ridge breach, the message went on to explain that "hackers potentially succeeded in gaining access to one of the laboratory's nonclassified databases that contained personal information of visitors to the laboratory between 1990 and 2004."
The personal information at risk includes names, dates of birth and Social Security numbers of the visitors.
"You would be amazed at the number of attempts we experience every week, both coordinated and uncoordinated, to penetrate networks," Homeland Security Secretary Michael Chertoff said Friday, "whether it is government agencies or private sector agencies."
Chertoff said that the department has been increasing its anti-hacking efforts over the past months. He said cyber security protection requires a multi-pronged approach, including building firewalls to prevent outside intrusion and increasing the funding of cyber security efforts, but that computer users also need to be mindful about opening suspicious emails.
As part of the hit on Oak Ridge, "thieves made approximately 1,100 attempts to steal data with a very sophisticated strategy that involved sending staff a total of seven 'phishing' e-mails, all of which at first glance appeared legitimate."
One of the fake e-mails appeared to be an announcement for a scientific conference; the other claimed it was a notice of a complaint on behalf of the Federal Trade Commission.
The lab's investigation found that approximately 11 employees took the bait and opened the e-mail attachments, "which enabled the hackers to infiltrate the system and remove data."
The sensitive Tennessee nuclear research facility has a staff of more than 4,200 and hosts approximately 3,000 guest researchers each year.
Mason said in the communiqué that the lab would attempt to contact the possible victims of the breach, but acknowledges that "the large number of out-of-date addresses will complicate this effort."
On a Web site the lab created to provide information to those at risk, a message recommends visitors place a fraud alert on their credit file, though Mason's message states that there is currently no evidence that any of the hacked information has been used.