STOCKHOLM, Sweden (AP) -- The Web site of Sweden's national police was shut down after a hacker attack that investigators on Friday said could be a retaliation for a crackdown on a popular file-sharing site called The Pirate Bay.

Meanwhile, the government faced allegations that police had acted under pressure from U.S. authorities when they targeted the file-sharing site in raids earlier this week.

The police Web site was closed late Thursday after a so-called denial of service attack, in which hackers overload a single network by directing massive traffic to the site, police spokesman Lars Lindahl said. He did not rule out that the attack was linked to the police crackdown on The Pirate Bay on Wednesday.

"It is quite possible, but that is only speculation," Lindahl said, adding a criminal investigation had been launched into the hacker attack.

In Wednesday's crackdown on illegal file-sharing, police said they raided about 10 locations in central Sweden and detained three men linked to The Pirate Bay on suspicions of violating copyright laws. The three Swedes were later released but could still face charges, Stockholm police spokesman Ulf Goranzon said.

The raids were applauded by the Motion Picture Association of America, which claims movie studios lost U.S. $6.1 billion (€4.7 billion) to piracy last year.

An opposition lawmaker called on Parliament's Constitution Committee to investigate the case after a report on public broadcaster SVT suggested the Justice Ministry ordered the raids on a U.S. request. Under Sweden's Constitution, ministers are not allowed to direct the work of the police.

"We want to find out whether pressure from the U.S. government was behind the action," Center Party spokesman Johan Linander was quoted as saying by newspaper Dagens Nyheter.

On Friday The Pirate Bay site displayed a message saying it would be "up and fully functional within a day or two."

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