A prosecutor says the gunman who attacked the high school in Parkland, Florida, in 2018 should be executed because he killed his victims in a cold and calculated manner.
Prosecutor Mike Satz told the 12 jurors who will decide whether Nikolas Cruz is sentenced to death or life in prison without parole how he killed each victim.
Cruz pleaded guilty in October to 17 counts of first-degree murder in the deaths of 14 students and three staff members at Marjory Stoneman Douglas High School.
The 23-year-old is contesting only his sentence.
The prosecution is expected to emphasize the cruelty and merciless nature of the shootings.
The defense will argue that the former Stoneman Douglas student suffered from mental and emotional problems his entire life and was abused.
One of the first to be called to the witness stand was junior Danielle Gilbert.
She was in a psychology class when the shooting began.
Gilbert told the courtroom that they "were sitting like sitting ducks" because they didn't have a way to protect themselves.
The case is expected to last for four months.
It is the nation’s deadliest mass shooting to go before a jury.