PEORIA, Illinois (Reuters) - An accused sleeper agent for al Qaeda labeled an “enemy combatant” and held in isolation in a U.S. Navy brig for six years pleaded guilty in court on Thursday to a terrorist conspiracy charge.

Suspected al Qaeda operative Ali al-Marri is seen in this undated photograph released to Reuters on January 6, 2009. REUTERS/Al-Marri family/Handout

Ali al-Marri, a 43-year-old with dual citizenship in Qatar and Saudi Arabia, could face up to 15 years in prison depending on whether U.S. District Court Judge Michael Mihm gives him credit for time served at sentencing on July 30.

Marri pleaded guilty to conspiracy to provide material support to al Qaeda. A second charge of providing material support for terrorism will be dropped based on a plea agreement that was finalized only minutes before Thursday’s hearing, Marri’s attorney said.

“The government would liked to have taken this to trial but I believe it was the right move for all,” Marri’s attorney Andrew Savage told reporters.

“Without a doubt, this case is a grim reminder of the seriousness of the threat we as a nation still face,” said Attorney General Eric Holder. “But it also reflects what we can achieve when we have faith in our criminal justice system.”

U.S. authorities said Marri had his first contacts with al Qaeda in 1998 and learned terror “tradecraft” through 2001 at the group’s military training camps in Pakistan.

There he met Khalid Sheikh Mohammed, the accused mastermind of the September 11 terror attacks, who directed him to meet with Mustafa al-Hawsawi, the suspected paymaster for the September 11 attackers. Hawsawi gave Marri $10,000, they said.

CODED EMAILS

The Justice Department said Marri communicated by email in code with Mohammed, whom he referred to as “Muk” while calling himself “Abdo,” and provided progress reports on his efforts to enter the United States.

Marri arrived with his family on a student visa on September 10, 2001, and went to Peoria, where he had previously been a student at Bradley University.

Prosecutors said he did not attend classes and instead used his new laptop computer to do research on cyanide compounds and sulfuric acid with the goal of creating a lethal gas.

They said he also collected information about U.S. dams and tunnels, using a computer program that permits the user to anonymously search websites.

Savage said Marri would “state unequivocally that he would never engage in any violent actions that would harm an innocent person.”

Authorities said Marri plotted to follow up the September 11 attacks with chemical or biological assaults and cyber-attacks on the U.S. financial system.

“Ali al-Marri was an al Qaeda ‘sleeper’ operative working on U.S. soil and directed by the chief planner of the 9/11 attacks,” said Arthur Cummings of the FBI’s National Security Branch. “Al-Marri researched the use of chemical weapons, potential targets and maximum casualties.”

ORIGINAL CHARGES DROPPED

Marri collected information about how to hack into protected computer systems, and obtained stolen credit card numbers and driver’s licenses, prosecutors said.

He was originally arrested in Peoria in December 2001 as a material witness in a New York investigation of the September 11 attacks. Marri was then returned to Peoria and charged with credit card fraud and lying to the FBI.

But the charges were dropped in 2003 and then-President George W. Bush declared him an “enemy combatant” and sent him to the Consolidated Naval Brig in South Carolina. He was held in the military prison without charge and in extreme isolation for nearly six years.

Following a review ordered by President Barack Obama, Marri’s case was transferred to the U.S. court system, and he was indicted in Illinois in February on terrorism charges.

Some legal experts have said Marri’s case offered a preview of how the administration plans to deal with more than 200 inmates of the prison in Guantanamo Bay, Cuba, if it is closed as planned. Holder was in Europe earlier this week, seeking help in relocating Guantanamo detainees.