Mr. Kamphuis later disavowed any direct role in the so-called distributed denial of service, or DDoS, attack, which spilled over from Spamhaus to affect other sites. He took to Facebook to inform the world that the flood of Internet traffic that threatened to cripple parts of the Web emanated from Stophaus, an ad-hoc, amorphous group set up in January with the aim to thwart Spamhaus, a company it claims uses its “tiny business to attempt to control the Internet through underhanded extortion tactics.” Stophaus, which lists no contact or location for the group, claims to have members in the United States, Canada, Russia, Ukraine, China and Western Europe.

Mr. Kamphuis said Stophaus was not a front for him; he is merely acting as a spokesman.

Nonetheless, the authorities are curious. The Dutch national prosecutor’s office said on Thursday that it had opened an investigation. Wim de Bruin, a spokesman for the agency, which is based in Rotterdam, said prosecutors were first trying to determine whether the DDoS attacks had originated in the Netherlands. Authorities in Britain and several other European countries are also looking into the matter.

Mr. Kamphuis, who is believed to be about 35, is singled out because of his vocal role. “For the Dutch Internet community, it’s very clear that he has a big role in this, even if there isn’t 100 percent airtight proof that he is behind it,” said J. P. Velders, a security specialist at the University of Amsterdam. “He could not be not involved. How much is he involved — that is for law enforcement to figure out and to act upon.”

Greenhost, a Dutch Internet hosting service, said in a detailed blog post that it had found the digital fingerprints of CB3ROB when it examined the rogue traffic that had been directed at Spamhaus.

Mr. Kamphuis created CB3ROB in 1996 and helped set up CyberBunker in 1999. From 1999 to 2001, he worked on the help desk at a Dutch Internet service provider, XS4ALL, according to one senior manager at the company who declined to be named, citing company policy. One co-worker said Mr. Kamphuis was constantly being reprimanded for hacking into his employer’s computer system. He was known for eccentric behavior; during a company trip to Berlin, the former co-worker said, Mr. Kamphuis refused to travel with his colleagues and rode alone in a bus.