American officials said Sunday they did not believe nine U.S. citizens were among 33 suspects detained on terrorism charges in Saudi Arabia over the past week, as reported by a Saudi newspaper. The English-language daily Saudi Gazette, citing an unnamed source, reported Sunday that four Americans were detained last Monday, followed by another five in subsequent days. Saudi authorities also detained 14 Saudis, three Yemenis, two Syrians, an Indonesian, a Filipino, a United Arab Emirates citizen, a Palestinian and a citizen of Kazakhstan, the report said.

Six U.S. officials told Reuters the U.S. government could not confirm that any Americans were among the 33 suspects detained.

However, two officials said U.S. authorities were still checking names against databases. Saudi authorities were also investigating the citizenship of those detained, one of the officials said.

None of the U.S. officials was authorized to speak publicly, and the U.S. embassy did not immediately respond to a request for comment.

Saudi Arabia in 2014 declared the Islamic State group a terrorist organisation and has detained hundreds of its supporters. The group, which controls territory in Iraq and Syria, has staged a series of attacks in the kingdom.

Friday, an attack at a Shiite Muslim mosque in Saudi Arabia’s al-Ahsa district in Eastern province killed four people and injured 18, the latest in a string of attacks claimed by Sunni jihadists that have left more than 50 dead in the past year.

The website of the Interior Ministry’s militant rehabilitation center listed four U.S. citizens as having been detained Jan. 25 and four more over the previous three months. It did not list any more recent detentions.

An Interior Ministry representative directed Reuters to the website, which gives information on all people detained as militant suspects, but gave no further comment.

The ministry Saturday identified one of the attackers in al-Ahsa as 22-year-old Abdulrahman al-Tuwaijri, a Saudi citizen, who detonated an explosive vest outside the Imam Rida mosque in the Mahasen district of Hofuf in al-Ahsa. A 27-year-old was also arrested wearing an explosive vest and carrying hand grenades when members of the mosque’s congregation seized him after he fired shots at them during the attack, the ministry said.

Attacks by supporters of the Islamic State group in Saudi Arabia include two bombings and two mass shootings at Shiite mosques. A mosque used by Sunni security services was also bombed.

The Saudi clergy have denounced the group as “kharijites,” an early Islamic sect reviled by Muslims for its extreme ideology.