BBC's Paul Royall: There is still no definitive explanation for their deaths

The British tourists and one Australian were killed as troops attempted an operation to rescue the 16 Western hostages.

Teacher Margaret Whitehouse, 52, from Hampshire, Ruth Williamson, 34, an NHS employee from Edinburgh, and university lecturer Peter Rowe, 60, from Durham, died along with Australian Andrew Thirsk, 35.

British detectives and agents from the American FBI are now investigating suspected links between the Islamic organisation behind the abduction, and groups in other countries.

British diplomats have expressed concern that the Yemeni authorities knew extremists were planning to attack a British target but failed to warn them.

Group has 'links to other extremists'

The UK Government has twice summoned the Yemeni ambassador to London meetings at the Foreign Office to press for a full, detailed account of the kidnapping and rescue operation.

Foreign Office Minister Joyce Quin says Britain is "seriously concerned" at the way the hostage crisis was handled.

Yemen says the kidnappers are part of a group calling itself Islamic Jihad and were already shooting hostages when the order was given to start the rescue. They say the group has links to other extremist groups outside Yemen.

The US suspects Osama bin Laden of bombing its embassies

Officers from the US and UK who travelled to Yemen are trying to determine whether the group is connected to the Saudi dissident, Osama bin Laden. The US believes he is the mastermind behind last year's bombings of their embassies in Tanzania and Kenya.

The UK newspaper, The Sunday Telegraph, reports that 10 US intelligence officers who have flown to Yemen believe the 16 tourists were kidnapped as "direct retribution" for last month's air attacks on Iraq.

The US, says the newspaper, suspects that the tourists may have been kidnapped to "shield" Saddam Hussein from further bombing raids.

Eight Britons who survived Tuesday's shoot-out arrived back at Gatwick Airport on Friday night, while two women, including Dr Rowe's wife Claire Marston, were too badly injured to fly home and remain in hospital.