FORT LAUDERDALE, FLA. -- The lead attorney for Florida school shooter Nikolas Cruz told the jury in his penalty trial Monday they have witnessed 鈥渢hings that will haunt them for the rest of their lives,鈥 but they now need to learn what led him to massacre of 17 people four years ago before they can decide whether he is sentenced to death or life without parole.

Melisa McNeill told the seven-man, five-woman jury and their 10 alternates in her opening statement that Cruz committed a horrible, unspeakable act when he murdered 14 students and three staff members at Parkland's Marjory Stoneman Douglas High School on Feb. 14, 2018, but he was a 鈥渄amaged person鈥 whose birth mother had abused alcohol and cocaine throughout her pregnancy.

鈥淲e must understand the person behind the crime,鈥 McNeill, an assistant public defender, said.

Cruz. 23, pleaded guilty in October to 17 counts of first-degree murder and his trial is only to decide whether he is sentenced to death or life without parole. This is the deadliest U.S. mass shooting to ever reach trial. Nine other gunmen who killed at least 17 people died during or immediately after their shootings, either by suicide or police gunfire. The suspect in the 2019 slaying of 23 people at a Walmart in El Paso, Texas, is awaiting trial.

鈥淚 stand before you today in a space filled with overwhelming sadness, painstaking grief, anger and trauma,鈥 McNeill said.

McNeill had deferred her opening statement from the trial's first day of July 18 to the beginning of her team's case. A rare strategy, it appears part of a broader plan to acknowledge the horror of Cruz's slaughter and not anger jurors by challenging it.

Instead, the defense will focus on what happened to Cruz over two decades to create someone capable of coldly shooting others during a seven-minute massacre, from his mother's drug and alcohol abuse during pregnancy to the bullying and alleged sexual abuse he endured. He exhibited severe mental and emotional problems starting in preschool, where he was kicked out for hurting other children.

During his public school years, he spent considerable time at a school for students with emotional problems before he was expelled from Stoneman Douglas about a year before the shooting.

Her team will call their first witness later Monday for a case that is expected to last several weeks. The trial had been on a two-week hiatus before resuming Monday.

During the prosecution's three-week case, the defense rarely cross-examined witnesses. They asked one teacher from a classroom where no one was shot about the lack of a security monitor in the three-story building where the slayings happened. When the gun store owner who sold Cruz the AR-15-style semiautomatic rifle used in the killings testified, they asked what Florida's minimum age was in 2017 to buy a rifle - 18 - and today - 21.

Neither they nor the prosecutors then asked the store owner why the law was changed: Cruz was 19 when the shooting happened and the Republican-led Legislature raised the age limit as part of a larger package of gun laws enacted in response to the shootings.

Cruz's youth will be part of his defense and while Circuit Judge Elizabeth Scherer has barred the defense from presenting testimony that directly casts blame on third parties like school administrators for not preventing the shooting, his attorneys will likely try to indirectly make such points.

The defense will be trying to overcome the horrendous evidence that was laid out by lead prosecutor Mike Satz and his team, capped by the jurors' Aug. 4 visit to the fenced-off building that Cruz stalked, firing about 150 shots down halls and into classrooms. The jurors saw dried blood on floors and walls, bullet holes in doors and windows and remnants of Valentine's Day balloons, flowers and cards.

Prosecutors also presented graphic surveillance videos of the massacre; gruesome crime scene and autopsy photos from its aftermath; emotional testimony from teachers and students who witnessed others die; and four days of tearful and angry statements from parents, spouses and other family members about the victims and how their loved one's death affected their lives. Jurors also watched video of Cruz calmly ordering a cherry and blue raspberry Icee minutes after the shooting and, nine months later, attacking a jail guard.

After the defense concludes its case, the prosecution will present a rebuttal case before each side gives its closing argument and jury deliberations begin.

Associated Press writer Freida Frisaro in Fort Lauderdale, Florida, contributed to this report.