BERLIN, June 2: Saudi Arabia struck a deal to free three hostage-takers in Saturday's suspected Al Qaeda attack because it feared accomplices could blow up an entire housing compound, a Saudi security adviser said on Tuesday.

Nawaf Obaid said the hostage-takers led authorities to believe the compound, in the eastern oil city of Al Khobar, was rigged with explosives and that they had fellow militants outside who could blow it up at the touch of a button.

"The threat was that if they killed (the hostage-takers), their so-called brothers from the outside would basically press the button and have the whole building go down. That's what they had threatened," Mr Obaid said in a telephone interview.

"It was on that premise that they agreed to do the bargain deal." His comments shed light on how, despite being surrounded by Saudi special forces, three of the militants were able to escape the scene of the hostage drama. A fourth gunman, the ringleader, was wounded and captured.

Twenty-two people, including 19 foreigners, were killed in the shooting and hostage-taking attack - the second major strike in a month on the Saudi oil industry. A purported Al Qaeda statement claimed responsibility for the attack in the world's top oil exporter that sent crude oil futures to a record of more than 42 dollars a barrel on Tuesday.

Mr Obaid describes himself as security consultant to a senior Saudi official. He is also co-author with US expert Anthony Cordesman, of the Center for Strategic and International Studies in Washington, of a new report entitled "Saudi Internal Security: A Risk Assessment".

He said the agreement to free the hostage-takers was approved at a high level. "It was a deal, and the orders came from senior people who said: 'Let them out'," he said. "It was basically a call between storming the compound and having more hostages die, or doing the bargain they did."

Mr Obaid said the compound was not in fact wired to explode, but security forces were only able to establish this as they moved through it, floor by floor. He ridiculed media reports suggesting the militants' escape implied Al Qaeda had penetrated the Saudi security services or was acting with their collusion.

Mr Obaid also rejected charges from expatriates that security around the Al Khobar compound was lax. He said the militants had planned to ram it with a car bomb but had been unable to get close enough. Instead, they launched their attack by killing security guards and scaling the walls, he said.

Mr Obaid said Saudi authorities were braced for further attacks: "There will definitely be more attempts, that's for sure." But he said successfully targeting Saudi Arabia's oil infrastructure was a "completely different ball game" for the militants.

"I'm sure ultimately they could have a try at it. I don't personally think from my assessment that they have the capabilities to do so," he said. Mr Obaid said there would be a further acceleration in training and equipping of Saudi counter-terrorist units, including the some 10,000-strong Special Emergency Forces leading the fight against Al Qaeda.

He said four European countries were carrying out such training, but declined to name them. US involvement was mainly with the Saudi armed forces, as opposed to the police and security services, Mr Obaid said. -Reuters