Saudi police have arrested four more suspects linked to Al Qaeda (search) as they crack down on extremists following the bombings in Riyadh, a Western diplomat said Thursday.
It was the first report of the arrests, which the diplomat said were made Tuesday. He had no details on where or how the arrests were made.
Saudi officials had announced Sunday that four suspects with apparent ties to Al Qaeda were arrested last week for the May 12 attacks on foreign housing complexes in Riyadh (search).
On Tuesday, Saudi security officials said three suspected Al Qaeda militants had been arrested a day earlier in the western port city of Jiddah. Later reports -- denied by the Saudi interior minister -- linked those three to a possible hijacking plot.
"The Saudis are working very, very aggressively throughout the kingdom," the Western diplomat said, speaking on condition he not be identified.
Saudi officials say they are taking action to avert more violence, but cannot guarantee that the attacks have ended. Security was tight Thursday in Riyadh, with some checkpoints creating traffic jams.
A 50-year-old Riyadh businessman who gave only his first name, Abdel-Karim, said he and his friends hesitated to go out due to fear and a reluctance to face the increased security.
"But we need the security," he said Thursday as he left a grocery store. "We are really scared right now. We don't know what is going to happen next."
Britain, Germany and Italy joined the United States in closing diplomatic offices in Saudi Arabia for at least a few days starting Wednesday because of terror fears. The United States also closed its embassy in the Norwegian capital of Oslo for security reasons Thursday after that country was singled out in a new statement attributed to Al Qaeda.
The diplomat said the latest arrests involved four men linked to, but not among, 19 suspects the Saudis have sought in connection with a weapons cache found in Riyadh on May 6.
The government had said the 19 were believed to be receiving orders directly from Al Qaeda leader Usama bin Laden (search), and had been planning to use the seized weapons to attack the Saudi royal family and American and British interests.
Less than a week after the weapons were found, attackers shot their way into three Riyadh compounds housing foreigners -- including one compound near where the weapons had been hidden -- and set off car bombs.
Thirty four people were killed in the 12 attacks, including eight Americans and nine attackers.
Interior Minister Prince Nayef has said investigators have identified three of the mangled bodies of the nine Saudi attackers as among the 19 suspects sought in the weapons cache discovery.
The diplomat said he could not confirm reports that the three suspects arrested Monday were Moroccan would-be hijackers and that their arrests thwarted a possible plan reminiscent of the Sept. 11 attacks on the United States.
The Saudi interior minister, Prince Nayef, was quoted as saying the report of a hijacking plot had "no basis in truth."
As Saudi officials cracked down on Al Qaeda sympathizers and possible operatives, the Arab satellite station al-Jazeera released an audiotape Wednesday in which a voice purported to be that of bin Laden's top lieutenant, Ayman al-Zawahri, called on Muslims to "consider your 19 brothers who attacked America in Washington and New York with their planes as an example."
The 19 Sept. 11 hijackers, who included 15 Saudis, slammed planes into New York's World Trade Center and the Pentagon in Washington.
In Washington, a U.S. official, speaking on condition of anonymity, said it was plausible that the speaker was al-Zawahri but a thorough analysis was necessary to be certain.
In the recording, the speaker lashed out at Arab governments that helped the United States in the war on Iraq.
The voice was similar to al-Zawahri's, based on previous audio- and videotapes attributed to the Egyptian militant. Al-Jazeera said it obtained the latest tape on Tuesday.
The voice on the audiotape also called for strikes on the United States and its allies in the war on Iraq, naming Britain, Australia and Norway.
Britain was America's main partner in the war on Iraq and Australia contributed troops.
Norway did not take part in the Iraq fighting, but provided special forces and other support in the war that dislodged Al Qaeda from Afghanistan and has been involved in mediating Mideast peace accords.