US attorney general Janet Reno launched a criminal probe by the Federal Bureau of Investigation (FBI) on Wednesday, following a third day of attacks by hackers on major Internet sites.
The latest targets were online brokerage, eTrade Group, which saw services at its website disrupted for 30 minutes, and technology news site, ZDNet, which was brought to a halt for two hours after hackers flooded it with fraudulent queries.
The attacks followed similar experiences at Yahoo, Amazon, eBay and other websites earlier in the week.
But Reno said the US government was committed to tracking down those responsible in order to bring them to justice and to ensure the law was enforced.
She vowed to take steps to make sure that cyberspace remained a secure place to do business so that the Internet could continue "to bring the world together rather than split it apart".
Federal prosecutors will join with the FBI's National Infrastructure Protection Centre to conduct the investigation, but the FBI warned that hackers could be using legitimate companies to launch their attacks and stressed that organisations needed to develop better security procedures.
The FBI also said it had not received claims of responsibility from anyone and had not yet managed to round up any suspects.
Under federal law, however, it is a crime to transmit software code or commands that are intended to damage websites or computers. First offenders can expect a prison term of up to five years, while subsequent offences may lead to sentences of up to 10 years.
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