MIAMI  One of seven indigent men charged with plotting to blow up the Sears Tower in Chicago as part of an Islamic jihad was acquitted on Thursday, and a mistrial was declared in the prosecution of the six others after the jury said it was hopelessly deadlocked.

The outcome was a significant defeat for the Bush administration, which had described the case as a major crackdown on homegrown terrorists.

Officials had acknowledged that the defendants, known as the Liberty City Seven for the depressed section of Miami where they frequently gathered in a rundown warehouse, had never acquired weapons or equipment and had posed no immediate threat. But, the officials said, the case underscored a need for pre-emptive terrorism prosecutions.

In acquitting Lyglenson Lemorin, 32, a Haitian immigrant who was cast by the prosecution as a junior foot soldier in the group, the jurors were compelled by evidence that suggested he had tried “to distance himself” from the others, Jeffrey Agron, the jury foreman, said outside the courthouse.