Authorities are interrogating Saud Abdulaziz Saud al-Rasheed "and if it is proven that he was connected to terrorism, he will be referred to the sharia (Islamic) court," the official Saudi Press Agency quoted an unidentified ministry official as saying.
In Washington, the FBI said it was cooperating with Saudi officials. "Our legal attache in Riyadh is working in concert with the Saudi officials and the State Department, and we can confirm he is in custody," said FBI spokeswoman Debbie Weierman.
On Saturday, al-Rasheed's father said his son had surrendered voluntarily to the Interior Ministry on Thursday in his hometown of Riyadh.
The ministry official said al-Rasheed has been in custody since Thursday, but did not say if he surrendered or was detained.
Initial investigations had shown al-Rasheed traveled to Afghanistan in June 2000 and returned to Saudi Arabia the following June, but that he had never traveled to America, the agency report said without elaborating further.
Al-Rasheed's father, who works for the Saudi Red Crescent in the capital Riyadh, said he had urged his son to surrender because he was sure of his innocence and feared for his safety after the FBI alert, issued Tuesday.
The elder al-Rasheed denied FBI accusations against his son, calling him a peaceful person who "has nothing to do with terror networks."
"He has never held a gun in his life," the father said.
He said his son was in Egypt when the alert was issued and returned to Saudi Arabia on Wednesday.
The FBI bulletin said al-Rasheed was suspected of being "associated with the September 11, 2001 hijackers" and warned that he should be considered armed and dangerous.
The alert was issued after an image of al-Rasheed's Saudi passport was found among material "previously recovered during the war on terrorism" connected to the Sept. 11 hijackers.
America has blamed the Pentagon and World Trade Center attacks on al Qaeda, the terror group led by Saudi exile Osama bin Laden.
Saudi Arabia — a key U.S. ally in the Gulf and home to 15 of the 19 Sept. 11 hijackers — has defended itself against accusations in the American media and policy circles that it is not doing enough to crack down on suspected militants within its borders.
Earlier this month, Saudi Foreign Minister Prince Saud al-Faisal said the kingdom was holding 16 suspected al Qaeda members transferred from Iran because they were Saudi nationals.
It wasn't clear if U.S. investigators would be allowed to interview the suspects or if they had sought permission to do so.
During an investigation into the 1996 Khobar Towers bombing that killed 19 U.S. servicemen, no American investigators were allowed access to suspects in Saudi jails.