Birthplace of Osama Bin Laden, and possibly a crucial part of the investigation. Fifteen of the 19 hijackers were Saudis, according to US federal investigators.
The complicated relationship between the US and Saudi Arabia has also played a critical part in the development of al-Qaeda.
Saudi Arabia is the world's largest producer of oil, and the US has poured military personnel into the country to protect its interests. The establishment of US air bases in Saudi Arabia was initially the main reason for Bin Laden's campaign against the US.
The Saudi regime stripped Bin Laden of his passport in 1994 but he still enjoys popular support. It is believed that he also receives financial backing from some Saudi organisations.
The Saudi Government has helped with the investigation, confirming the identities of the hijackers, and making hundreds of arrests. But the royal family are reluctant about being perceived as too cosy with US because of widespread opposition to the US-led war in Afghanistan.