WASHINGTON — A malware-laden e-mail masquerading as a White House Christmas card was a sinister move by hackers to steal sensitive documents from U.S. law enforcement and military officials, according to cybersecurity analysts.

The bright red and green holiday greeting, with the decorated Christmas tree, was sent out in late December and claimed to be from the "Executive Office of the President." Cyber threat analysts said it was targeted at government officials, particularly those who are involved in computer crime investigations.

While it is not clear yet how many people got the malicious e-mail or how many documents were siphoned from their infected computers, analysts said there has so far been no evidence that any classified data was taken.

The targeted e-mail attack comes as the federal government is desperately trying to beef up its cybersecurity after the release of thousands of State Department cables and military documents by the WikiLeaks website. Federal authorities want to improve technology systems and crack down on employees to prevent the theft or loss of classified and sensitive information.

A memo distributed this week by the White House Office of Management and Budget instructs federal agencies to complete assessments of system gaps and weaknesses as well as plans to upgrade networks and websites by Jan. 28. The agencies must detail whether they have adequate procedures for workers accessing classified materials, how they determine who is given that access, and whether they use psychiatrists or sociologists to measure if employees are happy or grumpy and could pose a security threat.

The e-mail prompted recipients to click on a link, which would then download the ZueS malware — a well-known malicious code that is often used to steal passwords and other online credentials, primarily to poach Internet banking information. The malware was created several years ago and is widely available for criminals to acquire and adapt. It has been used to steal millions of dollars.

In this case, however, the code downloaded a second malware that is designed to steal documents from the recipient’s computer, accessing Microsoft Word and Excel files.

Don Jackson, director of threat intelligence for Atlanta-based SecureWorks, a computer security consulting company, said the attack was somewhat small and targeted to a limited number of groups with law enforcement, military and government affiliations.

It was small enough, he said, to suggest that is was sent out manually and not by a large network of infected computers. He said it was not large enough to be picked up by cybersecurity spam traps or sensors.

Alex Cox, principle research analyst for NetWitness, a cybersecurity firm in northern Virginia, said the email was sent out just a day or so before Christmas, delivered by a control server in Belarus. He and Jackson said they believe this ZueS version was created by the same people who launched a similar but much larger attack last February.

Cox, who discovered the ZueS-infected malware last year when it infected at least 74,000 computers, said it’s hard to determine how many people were affected or how many documents were stolen in this latest attack.

Jackson said the hackers stole at least several gigabytes of data.

Analysts learned of the e-mail attack last week and have spoken with federal authorities about it.

Homeland Security Department spokeswoman Amy Kudwa said officials are aware of the ZueS e-mail and are monitoring it along with other similar malware attacks that have been tracked for some time.

Cox and Jackson would not disclose details on who was attacked or what documents may have been compromised but agreed that the hackers probably were after the documents, rather than any banking or financial passwords.

One theory, said Jackson, is that the hackers were looking for information about law enforcement cases and investigative techniques related to cybercrime so that they could sell it to other criminals.

The e-mail attack, however, underscores the continuing vulnerability of government workers and their computer systems to versions of the ZueS malware. Hackers can easily tweak the code each time so that it does not trigger antivirus software.

"Criminals have found that if they change the files in small ways it can slip past antivirus software," said Jackson.

While ZueS-related attacks are fairly common, this latest one stood out because of the use of the White House connection to lure recipients in and the targeted way it went after law enforcement, analysts said.

One U.S. official said the code was rather poorly written. The hackers could only get easily accessible documents and not those filed deep within layers of folders on the hard drive, said the official, who spoke on condition of anonymity to discuss ongoing investigations.