BOSTON — Some of the first prospective jurors questioned in the federal death penalty trial of Boston Marathon bombing suspect Dzhokhar Tsarnaev illustrated the difficulty in finding people who say they have not formed an opinion on Tsarnaev’s guilt.

As the second phase of jury selection began Thursday, the judge and attorneys heard from at least a half-dozen people who said they already believe Tsarnaev is guilty, several who said they could never impose the death penalty and several who said sitting on a long trial would be a serious financial hardship. The trial is expected to last three to four months.

They also heard from a man whose wife treated injured bombing victims.

Tsarnaev, 21, is accused in the deadly 2013 marathon bombing, which killed three people and injured more than 260. He is also accused of killing an MIT police officer days after the bombing.

The first juror questioned said he believes Tsarnaev is guilty and would sentence him to death. When pressed by Judge George O’Toole Jr. on whether he would be able to change his opinion after listening to evidence at the trial, the advertising account manager said, “I can’t imagine any evidence that would change how I feel about what happened.”

Tsarnaev listened to the man’s comments impassively while surrounded by his lawyers at a large table. At times, he appeared attentive and occasionally scribbled on a legal pad. Other times, he stroked his beard and looked around the room.

He had a neater, shorter haircut and a trimmed beard that contrasted to his appearance last week, when his hair appeared long and unkempt and his beard was scraggly.

One potential juror, a Catholic theologian, said he could not impose the death penalty under any circumstances. He later clarified that he could impose the death penalty only if there were no secure prisons to protect the public.

“There’s no way in modern America today … that I am going to vote for the death penalty. I will not,” he said.

Another man who was questioned said he works as a product manager at John Hancock, a major sponsor of the marathon. He also said his wife is an intensive care nurse who treated bombing victims.

“It’s tough, because it hit my wife hard,” he said, when asked if his wife’s experience would have an effect on his ability to be fair and impartial.

The judge questioned 20 prospective jurors Thursday, but it was not immediately clear how many of them would be excused. The judge huddled privately with lawyers late in the day and did not allow the media to be in the courtroom for the questioning. Reporters watched the questioning in an overflow courtroom through a video feed.

Some people told the judge that despite intense media coverage of the bombings, they had not formed an opinion on Tsarnaev’s guilt or innocence.

“I don’t know this guy over here from Adam,” said one man, a software programmer.

The man said he would base his decision on evidence presented at the trial.

“I don’t get swayed by the media,” he said. “I don’t get swayed by anybody.”

In initial instructions to a group of about 20 on Thursday morning, the judge took on a somber tone as he explained that the jury, after determining whether Tsarnaev is guilty, would also have to decide whether he is executed.

“The jury, and not the judge, is responsible for determining whether a defendant convicted of a capital crime will live or die,” O’Toole said.

Seventeen of the 30 charges against Tsarnaev are capital crimes.

A panel of 12 jurors and six alternates will be chosen. The judge has said he expects testimony in the trial to begin Jan. 26.

Last week, more than 1,350 people filled out lengthy juror questionnaires. Many were expected to be excused based on their responses, but the court has not yet released that number.

More prospective jurors will be questioned Friday.

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