Bern, Switzerland ? Swiss authorities said Friday they had detained five Islamic extremists suspected of using the Internet to show the killing of hostages — which reportedly included the beheading of an American — and to give bomb-making instructions.

It was unclear if any of the slayings took place in Iraq, where they have been blamed on the group led by Jordanian terrorist Abu Musab al-Zarqawi. Video recordings of those killings have been posted on various Islamic Web sites.

“The sites — which were actively exploited by at least one of the arrested persons — included numerous videos showing the putting to death of hostages and the mutilation of human beings,” said a statement by the Federal Prosecutor’s Office.

Peter Lehmann, spokesman for the office, declined to go into detail about the slayings, saying the locations of the killings had yet to be determined.

All the suspects were “of the Islamic faith, with extremist leanings,” the office said. It did not identify any of them, but said they came from Tunisia and Belgium and were legal residents of Switzerland.

They are under investigation for incitement to crime and for supporting a terrorist organization, it said.

The office said the forum of the now-closed Web site published letters claiming responsibility for a suicide bomb attack in July 2004 in Pakistan.

“The forum of one of the sites was often used by the Islamist movement as a communication and propaganda tool,” the statement said, adding that the investigation covered Arab-language sites.

The forum also contained threats against European governments and information relating to two French reporters, Christian Chesnot and Georges Malbrunot, who were held by Islamic militants in Iraq for four months before being released in December. The prosecutor’s office did not elaborate.

According to Swiss media reports after the site was shut down, its postings included a video showing the beheading of American Paul M. Johnson Jr., who was kidnapped and killed in June in Saudi Arabia, as well as a threat to kill Italian aid workers Simona Pari and Simona Torretta, who were abducted Sept. 7 in Baghdad and freed Sept. 28.

Other postings included a sermon by an Iraqi Islamic cleric urging Muslims to behead Jews and fight a holy war against unbelievers; pictures of terror mastermind Osama bin Laden fighting in Afghanistan; and graphic images of a man being beaten to death during Christian-Muslim violence in Indonesia.