From NBC's Doug Adams and Mike Isikoff
Last night, the hacker group called "Anonymous" brought down the Web site of Americans for Prosperity, the conservative advocacy group heavily funded by the billionaire brothers David and Charles Koch.

AFP has been very active in the Wisconsin standoff over collective bargaining rights,  spending more than $400,000 in TV ads in support of Gov. Scott Walker's plan.

Anonymous temporarily disabled the AFP site for a few hours through a DDOS or "distributed denial of service" attack. The group targeted AFP and the Koch brothers because of their support of Gov. Walker, saying in a statement: "Their actions to undermine the legitimate political process in Wisconsin are the final straw."

Anonymous has carried out dozens of computer attacks in recent months in defense of liberal causes, especially Wikileaks. Last December, the group brought down the web sites of Mastercard and Visa, because the companies had stopped processing donations for Wikileaks. The FBI is investigating the Mastercard/Visa attacks, and a grand jury in San Jose is considering charges this week.
 
On Friday, Anonymous pressured PayPal to back off its refusal to process donations for Army PFC Bradley Manning's legal defense fund. Last month, they hacked into the computers of a major cyber security firm, HB Gary, and published tens of thousands of embarrassing emails.

Here's the press release condemning the attack from Tim Phillips, the president of Americans for Prosperity

And here's the press release from Anonymous, calling for a boycott of Koch Industries and its affiliated paper company, Georgia Pacific.