Dutch authorities say police officials in Spain have arrested a man believed to be connected to an online attack on a spam-fighting site that snarled the Internet last month.

While the authorities did not give the full name of the man in a statement published on a Dutch government Web site, they identified him as “S.K.” A source close to the investigation, who was not authorized to speak publicly, confirmed that the arrested man was Sven Olaf Kamphuis, a 35-year-old Dutch man who has been the spokesman of a group that was protesting a European antispam group’s tactics.

Spanish police arrested the man on Thursday at his home in Barcelona, at the request of the Dutch police, and seized his computers and mobile phones. He is expected to be sent to the Netherlands. Wim de Bruin, a spokesman for Dutch national prosecutor’s office, said “S.K.” was suspected of playing a role in a wave of attacks last month.

His arrest came after an investigation by authorities in the Netherlands and other European countries into Mr. Kamphuis’s involvement in one of the largest attacks on the Internet. Mr. Kamphuis has been suspected of starting a distributed denial of service, or DDoS, attack against Spamhaus, the antispam group. Such attacks are a criminal offense under Dutch law.