Several government websites were forcibly shut down in Japan this week after hackers infiltrated the sites and altered content, reports the Asahi Shimbun.

The attacks were linked to international hacker group Anonymous, after it stated that it would launch cyber attacks on the Japanese government in protest against the reversion of the Anti-Piracy Law, which penalizes illegal music and video downloads from the Internet.

According to The Nikkei, The National Information Security Center detected unauthorized access on websites of the Finance Ministry, Supreme Court and the Democratic Party of Japan throughout Tuesday. Officials shut down the websites to avoid further infiltration.

However, it is still not clear whether Anonymous was behind the attacks. “We are looking into the cause of the trouble, including possible links with Anonymous, and measures to prevent a recurrence,” an official of the Fiscal Investment and Loan Management Division at the Finance Ministry’s Financial Bureau told Asahi Shimbun.

Anonymous protested against legislation that curb access to information and free speech, arguing that it “infringes on freedom on the Internet”.