RIYADH, Saudi Arabia -- FBI director Robert Mueller on Sunday said the Riyadh suicide bombings bore "all the hallmarks" of Osama bin Laden's al-Qaida terror group, and he praised the U.S.-Saudi cooperation in the investigation of the attacks.
Mueller, who arrived in Saudi Arabia earlier in the day from Jordan, said he was in the Gulf kingdom to praise Saudi officials for their support in the war on terror and the probe into the fatal May 12 attacks.
The State Department announced Sunday that a ninth American had died from injuries received in the late-night bombings at three Riyadh residential compounds. No details were released at the family's request.
Some 35 people are reported to have been killed, including nine bombers.
"I think we assume that, yes, there are ties to al-Qaida," Mueller said during a press conference about the attacks in the Saudi capital late Sunday.
"I don't want to get into the details because the investigations are ongoing, but clearly this type of undertaking has all the hallmarks of an al-Qaida operation."
A U.S. diplomat said Mueller planned to stay in Saudi Arabia through Monday. Moroccan officials in Rabat said the FBI chief is also expected to travel to Morocco within days for talks with King Mohammed VI.
Four days after the Riyadh attacks, the Moroccan city of Casablanca was rocked by nearly simultaneous bombings that killed 43 people, including 12 bombers.
The Casablanca bombing plotters are suspected of financial ties to senior al-Qaida leaders outside of Morocco, U.S. officials have said on condition of anonymity.
When asked Sunday if a link has been established between the Saudi and Moroccan attacks, Mueller told reporters: "It's too early to make any definite statement as to the relationships."
Mueller said since the Sept. 11, 2001, attacks in the United States and last month's Riyadh bombings, officials in both countries have been enhancing domestic security precautions, but more needed to be done.
Mueller would not disclose how many people have been arrested since the investigation into the Riyadh attacks was launched, but he said none of those in detention were thought to be on the FBI's most wanted terrorist list.
"(Our) Saudi counterparts have moved aggressively to identify and detain those who would have some participation, either directly or more remotely, to these events," he said.
Saudi authorities have detained about 100 people since the Riyadh bombings.
Late Sunday, an Interior Ministry official said 12 men and three women with rifles, bomb-making chemicals and $187,000 were arrested during the past two days in different locations in the holy city of Medina.
Ten of the detained men were Saudis; the others included Moroccans and a Syrian, authorities said. It was unclear if the suspects were connected to the May 12 attacks. The official only said they were detained in relation to the nationwide terrorism investigation.
Mueller said he was "tremendously gratified" by statements made by Saudi King Fahd and Crown Prince Abdullah following the May 12 attacks "expressing solidarity with (America) ... and others on the war against terrorism."
U.S. investigators are in Saudi Arabia to only "assist their Saudi counterparts at the three sites," Mueller said.
"We were pleased to be able to complement and help our Saudi counterparts in continuing ... gathering evidence at the site of the bombings," he said.
Only one team of U.S. investigators remains in Saudi Arabia. A team that arrived following the bombings has since returned to America. A smaller team of six or seven U.S. analysts and experts is expected to arrive soon.
After a 1996 truck bombing killed 19 U.S. military personnel in Saudi Arabia, U.S. officials complained about Saudis blocking access to evidence, witnesses and suspects.
In another development, one of 19 men sought over a May 6 weapons cache find in Riyadh was killed in a shootout with police on Saturday in northern Saudi Arabia.
Yosif Salih Fahd Ala'yeeri was among 19 men linked to the Riyadh attacks whose photographs were shown on Saudi TV. Saudi authorities have said the men were receiving orders directly from Saudi-born bin Laden.