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The Pirate Bay file-sharing site appears to be the subject of a deepening investigation by the Swedish authorities.

Binero, the Swedish company that hosts the site's web address, said that it had recently received a letter from the country's Prosecution Authority and police requesting information.

TPB's webmasters had earlier claimed that an inquiry initiated in 2010 had been.

The operators denied TPB was illegal.

TPB claims to have more than 5.5 million registered users. It offers Magnet links which provide users the ability to download copyrighted movies, music, books and computer applications.

In February the site's original administrators were told they could not appeal jail sentences and fines relating to their role in helping people circumvent copyright controls.

The site remained online as it was not part of the legal proceedings.

Shortly after the verdict was announced, the site's current operators switched it from a US-based .org address to a Sweden-based .se address.

They said they wanted to prevent the risk of the US authorities seizing the site, and to "make a statement" about their friends' prison sentences.

Binero's marketing manager, Erik Arnberg, told the ´óÏó´«Ã½ that his firm had been contacted in the latter half of February with a request to divulge information about a customer owning the domain names thepiratebay.se and piratebay.se.

"It said an investigation is being conducted, but there was no specific mention which crime was being investigated nor which law was applicable in this case apart from a general Swedish law which is a general requirement of any citizen to help the police with their questions," Mr Arnberg said.

"We do try to protect our customers' integrity, so in this case we have chosen not to answer all of the questions, but rather just forward onto them public information."

Public records only list thepiratebay.se's owner as "fredri4218-37643".

Mr Arnberg stressed that his company only hosted TPB's domain name and not any of the file-sharing links that it provided.

Neither the Swedish Prosecution Authority nor the Stockholm County Police could confirm the details of the inquiry when approached by the ´óÏó´«Ã½.

However, the Torrentfreak news site, which has closely followed the story, reports that the prosecution told its editor that it was "interested in torrent sites" without providing more detail.

Torrentfreak speculates that the authorities may be trying what they failed to achieve after a 2006 investigation: to".

TPB's webmasters say that their servers are set up in "a special way" so that they can never be pinpointed to one country.

"We think that the investigation is interesting considering nothing that TPB does is illegal," they wrote on its blog.

"We're staying put where we are. We're going nowhere."

However, users in the UK may find themselves unable to access the site later this year.

The High Court has ruled that it facilitates copyright infringement and will decide in June if internet service providers must block it.