US officials have largely ruled out North Korea as the origin of a computer attack last July that took down US and South Korean government websites, according to cybersecurity experts.

But authorities are not much closer than they were a year ago to knowing exactly who did it - and why.

In the days after the fast-moving, widespread attack, analysis pointed to North Korea as the likely starting point because code used in the attack included Korean language and other indicators. Experts now say there is no conclusive evidence that North Korea, or any other nation, orchestrated it.

The crippling strikes, known as "denial of service" attacks, did not compromise security or breach any sensitive data or critical systems. Officials and experts say the agencies are better prepared today. But they acknowledge that many government and business sites remain vulnerable to similar intrusions.

The incidents underscore the increasing threats posed by computer-based attacks, and how they can disrupt service as well as inflame political tensions.

Pinpointing the culprits for such attacks is difficult or even impossible, officials say. Some suggest the July 4 weekend attacks a year ago may have been designed as a political broadside.

These officials point suspicions at South Koreans, possibly activists, who are concerned about the threat from North Korea and would be looking to ramp up antagonism toward their neighbour. Several experts familiar with the investigation spoke on condition of anonymity because the results are not final.

According to US officials and private computer analysts, the attacks were largely restricted to vandalizing the public Web pages of about a half dozen federal agencies, including the Treasury Department and the Federal Trade Commission.

About three dozen other sites were targeted, including some private companies and a number of South Korean government sites, which reportedly had the most damage.

While the questions of who did it and why are unanswered, many investigators and experts now do not consider it a critical case.