Saudi security guards have thwarted an attempted attack on the Abqaiq oil facility. The BBC's security correspondent Frank Gardner explores some of the issues raised by the incident.
It is the first time attackers have targeted an oil facility
Q: What do we know about what happened?
This was a bold daytime attack on the world's largest oil-processing facility, called Abqaiq close to Saudi Arabia's main export terminals on the Gulf coast.
Two vehicles painted with the logo of the state oil company, Saudi Aramco, and packed with explosives, drew up.
They were challenged by guards, there was an exchange of gunfire and at least one of the vehicles blew up, killing the occupants.
Reports say there was then a gun fight with the surviving militants for about another two hours.
A nearby pipeline was reportedly damaged but the Saudi authorities were quick to insist that oil production was not affected.
Q: What was the effect of the attempted attack?
The net result was that the attackers failed to penetrate inside the oil facility, but the fact that it took place at all caused oil prices to jump on the news.
This is the first direct attack on oil facilities in the Gulf by militants, almost certainly linked to al-Qaeda.
There have been attacks in the past on Western expatriates working at oil and gas facilities in Saudi Arabia, but this was an actual attack on the physical facilities themselves.
Q: How significant is it?
For some months now, al-Qaeda's leaders including its second-in-command Ayman al-Zawahiri, have been calling on their followers to attack oil installations, saying that the revenues from Muslim oil go to the enemies of Islam.
They point out that Western governments place huge taxes on oil and therefore most of the money goes to Western governments rather than oil-producing countries in the Middle East.
But it is interesting that they should do this though, because if al-Qaeda really wanted to take over and eventually rule Saudi Arabia, attacking Saudi Arabia's natural resources would be counter-productive.
To really damage these oil exports and cripple the Western economies is very difficult to do.
That whole area is very spread out, so they would need to conduct a massive, co-ordinated attack on several facilities all at the same time and the Saudis seem to be alert to this ambition.
Security at these facilities is pretty good but it is unlikely to be the last time militants will try such an attack.
Q: Are the Saudis winning their war against al-Qaeda?
The Saudi authorities were initially reluctant to admit they had a problem with al-Qaeda militants on their soil but the suicide bomb attacks of May 2003 (that killed 35 people in Riyadh) changed that.
Since then they have fought a largely effective counter-terrorist campaign that has seen one al-Qaeda leader after another killed or captured, and the last major terrorist attack was in December 2004.
And yet clearly the al-Qaeda franchise in Saudi Arabia is more resilient than the authorities would like to admit.
It is not over yet and of course the big fear is that when the conflict in Iraq eventually dies down there will be some jihadis - volunteer fighters who have gone from Saudi Arabia to Iraq - who will then come back to Saudi Arabia, bloodied, violent and looking to carry out attacks in the country.
Their targets could be princes, Westerners, or oil facilities.