Ali al-Marri, 43, admitted to one count of conspiring to provide material support or resources to a foreign terrorist organization. A second charge of providing material support or resources to a foreign terrorist organization was dropped.
Al-Marri faces up to 15 years in prison and a $250,000 fine at his July 30 sentencing, though he will be credited for 18 months spent in civilian custody. His attorneys say they'll argue that he should get credit for the time spent in military custody, too - more than five years.
"That is an issue that has not been determined yet," Sharon Paul, a spokeswoman for the U.S. Attorney for the Central District of Illinois told CBS News.
"This will be a key issue at sentencing,"' al-Marri attorney Jonahtan Hafetz, of the ACLU told CBS News. "We will argue he deserves credit at least for every day and more given the brutality of his conditions."
In the case of one-time "enemy combatant" Jose Padilla, the federal judge factored the three-and-half-years he was harshly detained as an "enemy combatant" prior to his transfer back to the criminal justice system.
Padilla was sentenced in January 2008 to 17 years for his conviction on conspiracy to provide material support to a terrorist group, and he currently resides as the federal "Supermax" in Colorado. The government, which had asked for 30 years, has appealed his sentence as too light.
"Without a doubt, this case is a grim reminder of the seriousness of the threat we, as a nation, still face," U.S. Attorney General Eric Holder said in a statement Thursday. "But it also reflects what we can achieve when we have faith in our criminal justice system and are unwavering in our commitment to the values upon which this nation was founded and the rule of law."
Al-Marri's attorneys said their client, a married father of five from Qatar, chose to plead guilty to avoid the risk, if found guilty, of spending 30 years in prison.
"We thought (the plea) was the right approach to take based on the evidence the government allowed us to review over the last several weeks," attorney Andy Savage said outside the federal courthouse in Peoria.
When the judge asked al-Marri how would plead, the diminutive Bradley University graduate, seated at a table with his lawyers, paused briefly before answering without emotion, "guilty."
Al-Marri admitted he trained in al Qaeda camps and stayed in al Qaeda safe houses in Pakistan between 1998 and 2001, where he learned how to handle weapons and how to communicate by phone and e-mail using a code.
He also admitted meeting and having regular contact with Khalid Sheikh Mohammed, the alleged mastermind of the Sept. 11 attacks, and with Mustafa Ahmad al-Hawsawi, who allegedly helped the Sept. 11 hijackers with money and Western-style clothing.
Al-Marri, a legal U.S. resident, was arrested in late 2001 while studying at Bradley in Peoria after federal authorities alleged he was tied to organizers of the 2001 attacks.
The Bush administration declared al-Marri an "enemy combatant" in 2003 and held him without charges for more than five years at a Navy brig in South Carolina. His attorneys say he was tortured there.
The "enemy combatant" designation was dropped when he was indicted by a federal grand jury in Illinois. He was moved to a federal prison in Pekin, Ill., just outside Peoria, in March, and remains there.
Holder said President Barack Obama ordered him to review the al-Marri case shortly after Obama took office in January.
Al-Marri got a bachelor's degree in business management administration from Bradley in 1991, then went to work for a bank in Qatar. The government said he met with Osama bin Laden in the summer of 2001 and was sent to the U.S. to help al Qaeda operatives carry out post-Sept. 11, 2001, attacks.
Al-Marri obtained a student visa and returned to the U.S. the day before terrorists crashed two hijacked passenger planes into the World Trade Center and the Pentagon.