The verdict phase of the trial of the "Boston Bomber," Dzhokhar Tsarnaev, ended last week with pronouncements of "guilty" on all 30 of the charges against him. And those with some knowledge of the law know that the case now moves from the verdict phase of the trial to the sentencing phase where the death penalty is a real consideration - especially since 17 of the guilty verdicts against Tsarnaev are linked to the potential to impose the death penalty.

The sentencing phase of the trial, according to Boston Globe columnist Kevin Cullen, will begin on April 21, which just happens to be the day after this year's Boston Marathon - almost two years to the day of the bombing. According to Cullen, the gap between the verdict and the start of the sentencing phase is due to the defense's request for time to line up witnesses.

One might wonder what witnesses the defense could possibly call to "mitigate" what happened here. We will not know until after the prosecution has presented its evidence. Nevertheless, the sentencing phase of this trial will move forward much like the verdict phase. There will be openings, and then the prosecution will call witnesses and present evidence that will help its argument for the death penalty. Conversely, the defense will list mitigating factors in an attempt to secure a lesser sentence. 

The prosecution's "aggravating factors," per the law, can include the fact that deaths occurred during the commission of another crime, the fact there was a grave risk of death to additional persons, the heinous and cruel manner in which the crimes were committed, the fact that there was substantial planning, the vulnerability of the victims and the fact that there were multiple killings or attempted killings [See 18 U.S. Code Sec. 3592].

These are the aggravating factors that can be argued in this case, and the prosecution must prove them beyond a reasonable doubt. In addition, the prosecution can also submit evidence of factors not listed in the statute itself, such as a lack of remorse [See Barclay v. Florida 463 US 939 (1983)].  

The prosecution touched on many of these factors during the verdict phase of the trial, so we will see a great deal of repetition. We can also expect to see very dramatic and heart-wrenching victim impact statements. Given the number of people who lost limbs as a result of these crimes, and the number of people otherwise impacted by the death of loved ones, this evidence will likely be lengthy.

During Oklahoma bomber Timothy McVeigh's trial, the prosecution called 38 different witnesses to testify about the impact of his crimes on their lives. In fact, his defense argued that the sheer number of witnesses brought in was excessive, characterizing the tactic as "constitutionally intolerable." The court disagreed and ruled that such a number was proper under the circumstances [See U.S. v McVeigh 153 F3rd 1166 (10th Circ. 1998)].

Once the prosecution is done presenting aggravating factors, the defense team will have an opportunity to present their mitigating factors. Under the law these can include assertions of impaired capacity to appreciate the wrongfulness of his act, unusual and substantial duress, the fact that the defendant's participation was relatively "minor" - no prior criminal record - and any mental or emotional disturbance that the defendant may have suffered [See 18 U.S. Code Section 3592 (a)]. The defense must prove these factors by a "preponderance of the evidence," - a much lower standard than having to prove them "beyond a reasonable doubt."  

While we do not know for sure what mitigating evidence Tsarnaev's defense team will present, it's certain that much will hinge on his older brother, Tamerlan Tsarnaev, and the hold he held on the defendant. This was evident in the guilt phase of the trial where the defense called four witnesses. During this phase, the defense is likely to call many more to make their points heard in the sentencing phase.

Tsarnaev's youth will also be proposed as a mitigating factor. We saw that in the verdict phase as well, when his attorney, Judy Clarke, referred to him as "a teen doing teen things."

Clarke is a brilliant advocate who will do all she can to save her client's life - as she did in representing Jared Loughner, the man who killed six people and shot U.S. Rep. Gabby Giffords in 2011. As part of Loughner's defense, Clarke subpoenaed his family medical records going back to the late 1800s in an attempt to establish a genetics-based "inherited" mental condition as the cause for his crime. We may see something similar here - especially given Tsarnaev's troublesome family situation and recent scientific evidence that suggests such conditions can be attributed to or influenced by genetic factors.  

Imposing the death penalty must be a unanimous decision among jurors, but unanimity on any single mitigating factor is not necessary. This is an extremely confusing and difficult endeavor that often results in multiple appeals stretching all the way to the U.S. Supreme Court [See Weeks v. Angelone 528 US 225 (2000).]  

One the day after this year's Boston Marathon, the jury in this case will begin the process of determining whether to sentence Dzhokhar Tsarnaev to death. No matter the outcome, the decision will be a difficult one for jurors, who certainly deserve our admiration and respect for performing their civic duty so faithfully. These heavily burdened men and women join the survivors of this tragedy as vivid examples of what it means to be "Boston Strong."

Heather Hansen is a partner in the O'Brien and Ryan law firm who has been named one of the 50 top female lawyers in the state by Pennsylvania Super Lawyers. Heather is also a national television and radio legal analyst and journalist who has appeared extensively on CBS News, Fox News, Fox Business Channel,, CNN, HLN and Sirius XM radio. Her writing has appeared in Law360 and she has co-authored two chapters in medical texts regarding medical malpractice litigation. Follow her on Twitter at @imheatherhansen.