In the two years since twin bombs tore through crowds at the Boston Marathon finish line, the case against suspect Dzhokhar Tsarnaev has focused on arguments over where his trial should be held, who should sit on the jury and what evidence prosecutors should be allowed to use.

But the lenghty process of seating a jury concluded Tuesday, and starting today, the focus will shift dramatically to the harsh reality of what happened that day: the explosions, the screams, the chaos and the blood.

Prosecutors are expected to present graphic images of the carnage, including a surveillance video that authorities say shows Tsarnaev placing a backpack just feet from 8-year-old Martin Richard and his family. The boy died in the explosion.

The bombs set April 15, 2013, killed three people and injured more than 260. At least 16 lost limbs.

“When people start streaming into that courthouse — many with missing limbs — and the prosecutors get up off their chairs and start talking about this again, people are going to relive the enormity and the awful nature of this,” said Gerry Leone, a former state and federal prosecutor who led the prosecution of failed shoe bomber Richard Reid but is not involved in the Tsarnaev case.

“It wouldn’t surprise me to see the young boy’s parents as the first witnesses. Oftentimes, in a homicide case, you humanize the victims right away, and you’re brought right back to that day,” Leone said.

A blast from the second bomb killed Martin and tore off his 7-year-old sister’s left leg. Lingzi Lu, 23, a Boston University graduate student, was also killed by the blast. Krystal Campbell, a 29-year-old restaurant manager, was killed by the first bomb.

Authorities say Tsarnaev, then 19, and his older brother, Tamerlan, 26— ethnic Chechens who had lived in the volatile Dagestan region of Russia — carried out the bombings to retaliate against the U.S. for the wars in Iraq and Afghanistan. The brothers came to the U.S. with their parents about a decade before the bombings.

Tamerlan Tsarnaev died following a shootout with police several days after the bombings. Dzhokhar Tsarnaev, now 21, was captured and faces 30 federal charges in the bombings and in the fatal shooting of a Massachusetts Institute of Technology police officer while he was a fugitive. Seventeen of the charges carry the possibility of the death penalty.

Some bombing survivors have said they plan to attend the trial; others say they have no desire to be there.

“It’s not something I feel I need to do,” said Jarrod Clowery, who suffered burns and shrapnel wounds. “I have a second chance at life, and I’m living it.”

Clowery was watching the marathon with his friends, Paul and J.P. Norden, when the bombs exploded. The Nordens each lost a leg.

The Norden brothers also plan to stay away from the trial, but their mother, Liz, plans to be there every day.

“It’s important to me. I take it personally, what happened to my family,” she said.

Tsarnaev’s lawyers have made it clear that they plan to depict Tamerlan Tsarnaev as the mastermind of the attack and a powerful force in his brother’s life.

Legal analysts say portraying Tamerlan as a coercive influence will likely not be enough to win Dzhokhar an acquittal but could be a significant piece of the defense argument against the death penalty.