The attack is the first by militants on an economic target in the kingdom - a development that has long been feared by Riyadh, which has tightened security at oil installations since the upsurge in Islamist violence following the 11 September terrorist attacks and the US invasion of Iraq. Saudi Aramco said that measures have further strengthened after the Yanbu attack. In mid-April, the US took the unusual step of ordering all nonessential diplomatic staff and their families to leave the kingdom, citing intelligence suggesting imminent attacks. Days later, the security services' headquarters in Riyadh was hit by a car bomb attack.