Osama bin Laden's son-in-law pleaded not guilty Friday to plotting against Americans in his role as al-Qaeda's top spokesman as a landmark case trying a terror suspect on U.S. soil moves forward.

Sulaiman Abu Ghaith entered the plea through a lawyer to one count of conspiracy to kill Americans after being captured in Jordan over the past week.

The case marks a legal victory for the Obama administration, which has long sought to charge senior al-Qaeda suspects in U.S. federal courts instead of holding them at the military detention centre at Guantanamo Bay, Cuba. Charging foreign terror suspects in federal courts was a top pledge by President Barack Obama shortly after he took office in 2009 — aimed, in part, to close Guantanamo Bay.

Republicans against trials in U.S.

Republicans, however, have fought the White House to keep Guantanamo open, and bringing Abu Ghaith to New York led to an outcry. Republicans in Congress do not want high-threat terror suspects brought into the United States, fearing that outcomes in a civilian jury trial are too unpredictable, compared to a military trial.

Attorney General Eric Holder announced the capture of Abu Ghaith on Thursday, saying "no amount of distance or time will weaken our resolve to bring America's enemies to justice."

Holder reluctantly agreed in 2011 to try self-professed al-Qaeda mastermind Khalid Sheikh Mohammed in a Guantanamo Bay military court instead of a civilian court after a fierce Republican backlash.

The Justice Department said Abu Ghaith was the spokesman for al-Qaeda, working alongside bin Laden and current leader Ayman al-Zawahri, since at least May 2001. Abu Ghaith is a former mosque preacher and teacher and urged followers to swear allegiance to bin Laden, prosecutors said.

The day after the Sept. 11 attacks, prosecutors say he appeared with bin Laden and al-Zawahri and called on the "nation of Islam" to battle against Jews, Christians and Americans.

A "great army is gathering against you," Abu Ghaith said on Sept. 12, 2001, according to prosecutors.

Kuwait stripped him of his citizenship after the 2001 attacks. In 2002, under pressure as the U.S. military and CIA searched for bin Laden, prosecutors said Abu Ghaith was smuggled into Iran from Afghanistan.

Tom Lynch, a research fellow at the National Defence University in Washington, described Abu Ghaith as one of a small handful of senior al-Qaeda leaders "capable of getting the old band back together and postured for a round of real serious international terror."

"His capture and extradition not only allows the U.S. to hold — and perhaps try — a reputed al-Qaeda core survivor, further tarnishing the AQ core brand, but it also points to the dangers for those few remaining al-Qaeda core refugees," Lynch said.

Several Republican lawmakers said Abu Ghaith should be considered an enemy combatant and sent to Guantanamo, where he could be questioned more thoroughly than his lawyers likely would allow as a federal defendant on U.S. soil.

Generally, Guantanamo detainees have fewer legal rights and due process than they would have in a court in America but could potentially yield more information to prevent future threats.

Since 2001, 67 foreign terror suspects have been convicted in U.S. federal courts, according to watchdog group Human Rights First, which obtained the data from the Justice Department through a Freedom of Information Act request.

By comparison, of the thousands of detainees who were swept up shortly after the terror attacks and held at Guantanamo Bay, only seven were convicted by military tribunals held at the U.S. Navy base in Cuba, the watchdog group said. The vast majority have been sent back overseas, either for rehabilitation or continued detention and prosecution.