By Elizabeth Barber

BOSTON (Reuters) - Two decades have passed since Timothy McVeigh set off a truck bomb outside a federal office building in Oklahoma City, killing 168 people, but the memory remains vivid for Tonya McCabe, who sat on the jury that sentenced him to die.

"I still want to think the best of people, but there is a part of me that was changed," said McCabe, who was 24 in 1997 when Gulf War veteran McVeigh was sentenced to death. "That natural instinct I had to think that people had the best intentions; after the trial, I didn't feel that way anymore."

Jury members in the Boston Marathon bombing trial may be in for a similarly formative experience on Monday. That's when they will begin their formal deliberations over whether Dzhokhar Tsarnaev is guilty of killing three people and injuring 264 at the race's finish line on April 15, 2013, and fatally shooting a police officer three days later.

About six out of ten jurors who sit on capital trials describe the experience as "emotionally upsetting," according to a 2008 study by Michael Antonio, a professor of criminal justice at West Chester University of Pennsylvania.

More than a third of 1,198 jurors who sat on 353 capital juries also experienced physical symptoms of stress during or after the trials, he said.

"It's not an easy job," said Jim Manspeaker, who retired in 2003 from a 41-year-long career as clerk of the federal district court in Denver, where McVeigh's trial was held. "Jury duty is a big responsibility. There is no question."

GRAPHIC ACCOUNTS

During the 16 days of testimony, the 12 jurors and six alternates heard gripping, and sometimes graphic, accounts of how the twin pressure-cooker bombs ripped through the crowd at the marathon, causing 17 people to lose limbs.

They saw autopsy photos of the mangled bodies of restaurant manager Krystle Campbell, 29, and Chinese graduate student Lingzi Lu, 23.

They also heard from William Richard, who described making the decision to leave his wife with his dying son, 8-year-old Martin, so he could try to save their 7-year-old daughter, Jane.

The jurors will now decide whether Tsarnaev, 21, is guilty of the 30 criminal counts he faces. Their conclusion will likely be made easier by the fact that Tsarnaev's lawyers opened the trial on March 4 with the blunt admission: "It was him."

Defense lawyers contend that Tsarnaev's older brother, Tamerlan, who died following a gunfight with police three days after the attack, was the driving force behind the bombing.

But deciding guilt is only the first half of this jury's job. If they find Dzhokhar Tsarnaev guilty, they will then hear another round of testimony, before deciding whether to sentence him to life in prison without possibility of parole or to death.

The toll of sentencing a person to die can often hit jurors heaviest after their service ends, said West Chester University's Antonio.

"During the trial, it's a struggle, but they have the other jurors that they're doing this with," Antonio said. "It's after the trial that the reality hits. We have lots of accounts of people who get to the parking lot, and then it hits them, 'Wow, I just sentenced someone to death.'"

EMOTIONAL TOLL

The sentencing decision is likely to be the most controversial part of the case. Thousands of Boston residents were crowded around the finish line when the bombs went off and hundreds of thousands hid in their homes four days later, during a massive manhunt for the brothers.

While anger runs high against Tsarnaev, the death penalty remains unpopular in liberal-leaning Boston.

The jurors will be "criticized no matter what they do," Antonio said.

After troubling trials, some courts bring in counselors, or, as in the McVeigh case, have judges de-brief the jurors.

But offering such assistance is not routine, said Janvier Slick, a clinical social worker hired by some trial courts in Oregon to counsel jurors after difficult cases, including the trial of Christian Longo, whom a jury sentenced to death in 2003 for the murder of his wife and three young children.

"Usually, when the jurors are dismissed, that's that," said Slick. "The court is done with them."

An official at Boston federal court said that Tsarnaev jurors would be offered counseling after the trial by the U.S. Public Health Service.

"The District of Massachusetts is prepared to offer these services following the completion of the trial," the official said.

McCabe, now 42, recalled that all 12 jurors in the McVeigh case cried in the deliberation room.

"It's like if we could say he wasn't guilty, then it wouldn't have been true and the bombings would have never happened," recalled McCabe, who changed her last name from Stedman after getting married 12 years ago.

Now, 18 years on, she is starting to put the experience behind her.

"The trial didn't disturb me to the core," said the mother of three who works in medical sales. "I'm not one to dwell."

(Editing by Scott Malone and Bernadette Baum)