By Frank Gardner BBC Security correspondent in Washington

Saudi Arabia's announcement on Tuesday that it had broken up a suspected al-Qaeda plot shows that the organisation is a serious threat inside the country.

The Saudi authorities said the group, led by a Sudanese man, had been planning bomb and missile attacks there.

intelligence analysts believed that it would be only a matter of time before al-Qaeda... struck at targets inside Saudi Arabia

Parts of a surface-to-air missile have been found near an airbase used by thousands of US airmen.

So clearly the Saudi Government is facing a bigger problem with al-Qaeda than it would like to admit.

Only a few days ago the Interior Ministry insisted there were no al-Qaeda sleeper cells in the country.

But now that appears to be wishful thinking.

Even if the hardcore members of al-Qaeda are no longer living there, their supporters or affiliates do have the will and the means to mount military operations inside the country.

The fact that this one seems to have been thwarted will come as little comfort to the ruling Saudi princes.

Sympathy

There are close to 15 million Saudis in the country and the security services cannot watch everyone all the time.

Complicating matters for the Saudi authorities is that many of their law-abiding citizens share some of al-Qaeda's views, but not their violent methods.

The vast majority of Saudis are angry at US policies in the Middle East, especially Washington's support for Israel.

At street level, many agree with al-Qaeda's conspiracy theory that the West is somehow out to suppress Muslims and Arabs.

But al-Qaeda's embrace of the Palestinian cause is relatively recent.

For the roots of its ideology we have to look further back.

Gulf War effect

When Osama Bin Laden went off to Afghanistan the first time, to fight the Soviets in the 1980's, he recruited other devout Saudi Muslims.

They returned as heroes, but were soon appalled to find their country playing host to half a million Western servicemen and women, preparing to drive Iraq out of Kuwait.

Illegal tapes of fiery sermons circulated round the country, damning the "unbelieving invaders".

When the US-led coalition won the Gulf War, devout Saudis hoped the Western troops would all leave.

But thousands stayed on at the invitation of the Saudi princes, flying daily air patrols over southern Iraq.

It soon became obvious to Osama Bin Laden and his followers that the Saudi Arabia of the 1990s was very far from their purist vision of an Islamic state.

As well as the "pollution" of holy Muslim soil by Christians and Jews, they railed against what they saw as the corrupt ways of their rulers.

Osama Bin Laden left the country and began to plot against the West.

Inevitable target

First from Sudan and then from Afghanistan, he built up the network that al-Qaeda is today.

In the past four years its most spectacular strikes have all been against the West.

But intelligence analysts believed that it would be only a matter of time before al-Qaeda turned its sights closer to home and struck at targets inside Saudi Arabia.

The discovery of a plot to attack key Saudi installations is not a major breakthrough in the War on al-Qaeda.

Compared to the big hits such as 11 September and the East African embassy bombings, this was a relatively small operation.

But had it succeeded in bringing down a US warplane the psychological boost to al-Qaeda would have been huge. Its supporters may well try again.