Story Highlights• Suspects believed to be affiliated with al Qaeda, official says
• Official: Roundup conducted just before expected attack
• Suspects are from several Arab nations
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RIYADH, Saudi Arabia (CNN) -- Saudi security officials said Saturday they foiled a planned terrorist suicide attack and arrested 139 suspected Islamist militants who were in "sleeper cells" believed to be affiliated with al Qaeda.
A senior official in the Saudi Interior Ministry told CNN that the suspects, who are from several Arab nations, were monitored by Saudi security agents for several months. They rounded the men up just before the expected attack was launched.
The suspects, arrested in different areas of Saudi Arabia, were being interrogated Saturday, the official said.
In October 2005, King Abdullah of Saudi Arabia called the threat of al Qaeda in his country "madness and evil" and vowed to "eliminate this scourge" of terrorism. Fifteen of the 19 al Qaeda terrorists participating in the September 11, 2001, attacks were Saudis.
There has been a series of deadly bombings in Saudi Arabia since then, including Western targets, further aligning the Saudi government with the United States in the war against terror.
In 2003, suicide bombings at residential compounds housing Westerners in Riyadh killed 23 people, and a car bombing in a mostly Arab neighborhood near Riyadh's diplomatic quarter killed at least 17. Both attacks were blamed on al Qaeda.
In December 2004 a Saudi-based al Qaeda group claimed responsibility for an armed raid on the U.S. consulate in Jeddah that left nine people dead.
Last year Saudi security forces, carrying out raids targeting suspected al Qaeda members, killed a man in Medina they said was the leader of al Qaeda on the Arab Peninsula.
CNN's Caroline Faraj in Dubai contributed to this report.