Ahmed Khalfan Ghailani faces major prison time despite being acquitted on more than 280 charges. (Associated Press)

The first Guantanamo detainee to face a civilian trial was acquitted Wednesday of all but one of the hundreds of charges alleging he helped unleash death and destruction on two U.S. embassies in 1998 — a mixed result for what's been viewed as a terror test case.

A federal jury convicted Ahmed Ghailani of one count of conspiracy to destroy U.S. property and acquitted him on more than 280 other counts, including one murder count for each of the 224 people killed in the embassy bombings. The anonymous jurors deliberated over seven days.

Ghailani, 36, rubbed his face, smiled and hugged his lawyers after the jurors filed out of the courtroom.

U.S. District Judge Lewis Kaplan had thanked the jury, saying the outcome showed that justice "can be rendered calmly, deliberately and fairly by ordinary people — people who are not beholden to any government, even this one."

Peter Quijano addresses the media after his client, Ahmed Ghailani, the first Guantanamo detainee to face a civilian trial, was acquitted on Wednesday of all but one charge accusing him of a deadly 1998 plot to bomb two U.S. embassies in Africa. Henny Ray Abrams)

In a statement, Department of Justice spokesman Matthew Miller said U.S. officials "respect the jury's verdict" and are "pleased" that Ghailani faces a minimum of 20 years and a maximum of life in prison at sentencing on Jan. 25.

Defence attorney Peter Quijano welcomed the acquittals. He said the one conviction would be appealed.

"We still truly believe he is innocent of all these charges," Quijano said. Still, Ghailani, who could have faced a mandatory life sentence if convicted of some of the other counts, "believed he got a fair trial," he added.

Prosecutors had branded Ghailani a cold-blooded terrorist. The defence portrayed him as a clueless errand boy, exploited by senior al-Qaeda operatives and framed by evidence from contaminated crime scenes.

Trial seen as test

The trial at a lower Manhattan courthouse had been viewed as a test for President Barack Obama administration's aim of putting other terror detainees — including self-professed Sept. 11 mastermind Khalid Sheik Mohammed and four other terrorism suspects held at Guantanamo Bay, Cuba — on trial on U.S. soil.

Ghailani's prosecution also demonstrated some of the constitutional challenges the government would face if that happens. On the eve of his trial last month, the judge barred the government from calling a key witness because the witness had been identified while Ghailani was being held at a secret CIA prison where harsh interrogation techniques were used.

After briefly considering an appeal of that ruling, prosecutors forged ahead with a case honed a decade ago in the prosecution of four other men charged in the same attacks in Tanzania and Kenya. All were convicted in the same courthouse and sentenced to life terms.

Prosecutors had alleged Ghailani helped an al-Qaeda cell buy a truck and components for explosives used in a suicide bombing in his native Tanzania on Aug. 7, 1998. The attack in Dar es Salaam and a nearly simultaneous bombing in Nairobi, Kenya, killed 12 Americans.

The day before the bombings, Ghailani boarded a one-way flight to Pakistan under an alias, prosecutors said. While on the run, he spent time in Afghanistan as a cook and bodyguard for Osama bin Laden and later as a document forger for al-Qaeda, authorities said.

Captured in Pakistan

He was captured in 2004 in Pakistan and was held by the CIA at a secret overseas camp. In 2006, he was transferred to Guantanamo and held until the decision last year to bring him to New York.

Despite losing its key witness, the government was given broad latitude to reference al-Qaeda and bin Laden. It did — again and again.

"This is Ahmed Ghailani. This is al-Qaeda. This is a terrorist. This is a killer," Assistant U.S. Attorney Harry Chernoff said in closing arguments.

The jury heard a former al-Qaeda member who has co-operated with the government describe how bin Laden took the group in a more radical direction with a 1998 fatwa, or religious edict, against Americans.

Bin Laden accused the United States of killing innocent women and children in the Middle East and decided "we should do the same," L'Houssaine Kherchtou said on the witness stand.

A prosecutor read aloud the fatwa, which called on Muslims to rise up and "kill the Americans and plunder their money wherever and whenever they can find it."

Other witnesses described how Ghailani bought gas tanks used in the truck bomb with cash supplied by the terror group, how the FBI found a blasting cap stashed in his room at a cell hideout and how he lied to family members about his escape, telling them he was going to Yemen to start a new life.

The defence never contested that Ghailani knew some of the plotters, but it claimed he was in the dark about their sinister intentions.

"Call him a fall guy, call him a pawn," Quijano said in his closing argument. "But don't call him guilty."

Quijano argued the investigation in Africa was too chaotic to produce reliable evidence. He said local authorities and the FBI "trampled all over" unsecured crime scenes during searches in Tanzania.