The family of a British oil executive shot dead by Al Qaida terrorists has accused the Saudi Arabian authorities of colluding with his killers to help them escape.

Four Islamic militants responsible for the shooting of 61-year-old Michael Hamilton, from Rye, holed up in an oil firm building after 21 other innocent victims were killed.

Saudi security forces besieged the building but three of the men escaped.

The Metropolitan Police, which is investigating, yesterday suggested there had been some kind of "understanding" between the security forces and the terrorists.

And Mr Hamilton's brother Douglas said the men escaped "purely because of their connections with the internal security of Saudi".

He added: "The thought that 40 of the special security people went in through various doors and in through the roof, and they could not contain four men, appears to be totally beyond belief.

"There are a lot of questions that have been asked. No one appears to have given a reasonable answer. It's very, very poor."

Mr Hamilton's widow Penelope said of the attackers: "I think they are evil, I think they are sick.

"They are doing their country no good. They are not doing Islam any good. They are not true Muslims. I have nothing but disgust for them.

"It's been a terrible time for us. I still can't really come to terms with the fact it happened and I no longer have a husband.

"He was completely innocent. He despised any kind of treachery or any kind of terrorism. He was a friend of the Middle East and a friend of Saudi Arabia.

"We all knew Al Qaida were here, we all realised there was a risk. But it was a risk you anticipated and hoped would never happen."

Mr Hamilton, who worked for the Arab Petroleum Investments Corporation, had just dropped off his wife at a nearby residential compound when he was shot nine times on May 29.

His body was tied to the back of his killers' car and dragged through the streets for more than a mile.

The terrorists later fired at a minibus full of children blocking the corporation's entrance gate, killing a ten-year-old Egyptian girl.

A second attack was launched by a separate group of militants at another petroleum compound three miles away, where several Westerners and Muslim staff members were executed.

Then the gunmen who killed Mr Hamilton are thought to have occupied a third compound in the city, taking hostages in a 12-hour siege.

Last month a verdict of unlawful killing was recorded at an inquest in Hastings into Mr Hamilton's death.

Sherard Cowper-Coles, Britain's ambassador to Riyadh, said the Met were helping investigate the "very grave allegation" about possible collusion.

He said: "It is impossible to imagine the terrorists escaping without some sort of understanding with the security forces.

"One cannot be clear exactly what happened. We are still piecing it together. The Metropolitan Police haven't finalised their internal report."

Jamal Khashoggi, media advisor to the Saudi ambassador to London, insisted the security commanders were concerned about protecting hostages.

He said: "A number of those terrorists were killed a couple of weeks later. One more handed himself in a week ago. They were pursued. The security went after them and are still going after them."

Mr Hamilton had worked in Al Khobar since 1989 but made regular returns to his British home in Rye where he and his wife planned to retire soon.