A jury in Miami has cleared one man of trying to blow up America's tallest building, the Sears Tower in Chicago, as part of a holy war.
The defence argued that the FBI had entrapped the defendants
The jury was unable to reach a verdict on six other defendants, and the judge declared a mistrial. Prosecutors say they plan to try them again next year.
The jury spent more than week considering their verdicts.
The defendants were known as the Liberty City 7 after the poor area of Miami they operated in.
The acquitted man, Lyglenson Lemorin, buried his face in his hands when the verdict came in.
The government had presented the arrests of the defendants as an important victory in its war on terror.
Al Qaeda oath
They faced up to 70 years in jail after facing charges on four counts, including taking an oath of allegiance to al-Qaeda.
The prosecution said they had hoped to forge an alliance with al-Qaeda to blow up the Sears Tower in Chicago and FBI offices in Miami and elsewhere.
But they succeeded only in contacting a paid FBI informant, rather than al-Qaeda itself.
And the defence argued that that they were in fact hapless figures who were either entrapped by the FBI or went along with the plot in order to con the FBI informant out of $50,000 (£25,000).
It said they wanted money to transform the warehouse in Liberty City where they met into a community gathering place.
Even the government described the alleged plot as more aspirational than operational.