"There are too many ifs, too many things going on," one male juror said. "I don't know the whole story."
Others say they just don't pay close enough attention to world events to be certain.
"I'm oblivious to that stuff," one prospective female juror said during questioning this week. "I don't watch the news much. I try to avoid it."
The doubts were noted by a significant portion of the more than 160 people who have been questioned individually since jury selection in the case began April 16.
Padilla and two co-defendants are charged with being part of a North American support cell for Islamic extremists. A jury is expected to be seated next week, with testimony to begin May 14.
Padilla, a U.S. citizen held for 3½ years as an enemy combatant, is accused of applying for an al Qaeda training camp in Afghanistan. He was previously accused of an al Qaeda plot to detonate a radioactive "dirty bomb" in a U.S. city, but that allegation is not part of the Miami case.
Before they came to court, each of the jurors filled out a 115-question form asking about a wide range of legal, political and religious topics, particularly their views of Arabs, Muslims and Islamic radicals. On question No. 60, which asks for an opinion about responsibility for the Sept. 11 terror attacks, many people said they don't know.
"I've been surprised at the number of our jurors who don't have an opinion about 9/11," U.S. District Judge Marcia Cooke, who is presiding over the case and asks most of the juror questions, said Wednesday.
The questionnaires were used to weed out dozens of people with obvious biases or personal hardships before the face-to-face interviews began, meaning many potential jurors with strong views about Sept. 11 never made it to court because their ability to be impartial was in question.
A cottage industry of conspiracy theorists has sprung up among academics and others who claim such things as that the U.S. was involved in the Sept. 11 attacks, or that explosives planted inside the World Trade Center towers brought the buildings down rather than the jetliners that crashed into them.
In the Padilla case, what's notable is not so much conspiracy theories as the lack of any views at all.
To be sure, most jurors without a Sept. 11 opinion are aware that the attacks have been blamed on terrorists of some sort. But many seem unwilling to blame al Qaeda and its leader, Osama bin Laden — the conclusion reached by the national Sept. 11 Commission and the Bush administration and widely reported by news media.
One female juror agreed that was a "general public consensus" but still held out skepticism.
"I don't have an opinion. I don't tend to trust the news media," she said.
Many jurors seem to be unwilling to state the al Qaeda connection as fact because they don't have firsthand knowledge. An older male juror said he answered "al Qaeda and bin Laden" on his questionnaire because "that was what the news said."
"I really can't say who did it," said the man, who was not being identified because Cooke has prohibited publication of jurors' names.
Samuel Terilli, a journalism professor at the University of Miami and former general counsel at The Miami Herald, said that hesitancy often comes naturally when people are asked for their opinions in an official setting, such as federal court.
"You have a tendency among some people when they are called to jury duty to heighten their skepticism about what they have read or watched, and also they have a desire to be more neutral," Terilli said. "People are on guard too much."
Some people say they don't necessarily believe the U.S. government's statements about Sept. 11, with many of those people citing the faulty intelligence and misinformation about weapons of mass destruction that led to the U.S. invasion of Iraq and the toppling of President Saddam Hussein.
"It could have been Saddam Hussein. It could have been bin Laden. I really don't know who," one woman said.