The group was split 11 to 1, he said, in favor of convicting Mr. Blagojevich on charges that he tried to trade or sell the Senate seat, but was divided in various ways at various moments on the rest of the counts. On some of the charges, said Mr. Sarnello, who is 21 and a student, the jury was divided evenly. “We were all over the place,” he said.

Mr. Sarnello said that he favored convicting Mr. Blagojevich on most counts, but that others raised a variety of concerns about whether he could be found guilty beyond a reasonable doubt.

Some of the jurors suggested that Mr. Blagojevich and his aides had been “just doing a lot of talking,” he said, but not actually committing crimes, while others felt that the government was too eager to “take him down,” or that his behavior might not be all that different from that of other politicians. Others expressed concern that some witnesses who testified against him had plea agreements or other deals with the government, deals these jurors said made their testimony suspect.

“Everybody in that room wanted to get the job done, but we came to the realization that it just wasn’t going to happen,” Mr. Sarnello said. “Some people just said they wanted clear-cut evidence  something clearer than they already had.”

After nearly two months of testimony and several weeks of deliberation, the overall result here was seen as a significant setback for federal authorities, who arrested Mr. Blagojevich almost two years ago to stop what they described unambiguously as “a political corruption crime spree.”

Patrick J. Fitzgerald, the United States attorney, declined to answer questions, but he gave a brief statement calling for respect for the jurors and their service, then added, “We’re about to get ready for a retrial.”

A spokesman for Mr. Fitzgerald said he could not comment on the cost of the prosecution, an effort that had spanned several years.