LONDON (AP) _ Three British survivors of a bloody shootout between their kidnappers and Yemeni troops have arrived home, saying they do not know who fired first during a rescue attempt that left four hostages dead.

Yemeni officials insist the kidnappers began executing their 16 hostages, prompting troops to open fire. Some of the hostages have said the soldiers attacked first, and the kidnappers tried to use their captives as human shields before killing them.

``None of us have any idea who fired first,'' said Brian Smith, who along with two other former hostages, spoke with reporters at London's Gatwick Airport on Friday. ``This is a question that has been fired at us time and time again and the simple answer is we do not know.''

The three arrived in London with six other hostages _ five British and one Australian _ flying from Yemen to Britain via Paris with a British tour leader who escaped the abduction Monday.

Smith and former hostages David Holmes and Sue Mattocks said they had not been pressured to change their stories to clear Yemeni authorities of blame, as has been reported.

However, when asked whether any other former hostages had been asked to do so, the three remained silent and refused to answer when the question was posed again.

The bodies of the four hostages killed _ Ruth Williamson, 34, Peter Rowe, 60, and Margaret Whitehouse, 52, of Britain, and Andrew Thirsk, 35, of Sydney, Australia _ remained in Yemen. Dr. Charles Rosenfarb of the U.S. Embassy said they might be flown to Britain on Sunday.

Three of the kidnappers also died in the rescue operation.

To clear up questions over how the hostages were killed, a representative of Britain's Foreign Office met for a second time with the Yemeni ambassador in London.

``The British government is seriously concerned about some of the aspects of the Yemen government's handling of the kidnap incident and we are very keen to establish the facts of what happened,'' a British Foreign Office spokeswoman said on condition of anonymity.

U.S. security experts and British detectives from Scotland Yard arrived in Yemen late Friday to investigate the deaths, a Yemeni security official said.

Mattocks said she had no quarrel with the Yemeni authorities.

``I personally feel that an operation like this is never going to be tidy,'' she said. ``I was concerned over the last couple of days about the sort of anti-Yemeni feeling. I don't feel that. I think that Yemen is a sovereign country and they made their decision.''

When the freed hostages began their journey home, a delegation of Yemeni officials bade them farewell. Young girls in national costume presented them with flowers and trade unionists held a banner saying: ``We are sorry. This is not like Yemen. Yemen is a friendly country.''

The killings have shocked Yemen, known for its hospitality even by kidnappers. In recent years, tribesmen seeking better roads or schools for their impoverished provinces have kidnapped scores of tourists but usually released them unharmed after treating them as guests. Until Tuesday, no hostage had been killed.