Spanish authorities would not confirm that Anonymous was behind the attack. Anonymous wasn't modest, however, posting information about the hack. The group called the operation Operation Policia (#OpPolicia), and said they used a DDoS (distributed denial of service) attack to bring the site down.
[Basically, a DDoS attack bombards a site with so many requests that it folds under the pressure. It is "distributed" because a multitude of systems are used to perform the attack.]
Anonymous claimed the DDoS attack was a form of peaceful protest. The group compared it to "sitting peacefully in front of a shop denying entry." Additionally, while Spain had earlier stated the three Anonymous hackers were leaders of the group, Anonymous noted that the group is loosely tied, and has no such leadership.