Jurors heard violent descriptions from inside the school during the rampage six years ago, including from an officer shot on that day.
The criminal trial has been on hold because the accused shooter, Dimitrios Pagourtzis, has been found incompetent to stand trial.
Thursday was the first time many family members have heard witness accounts.
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"We want people to hear what we have to say, and the evidence speaks for itself," Scot Rice, whose wife was shot, said.
The first to take the stand was a freshman at Santa Fe High School who was hiding inside a closet as bullets flew.
She testified that she called her mother and police as the accused shooter was shooting her classmates while singing "Another One Bites the Dust" by British rock band Queen.
Next, law enforcement officers who responded to the horrific scene testified.
A deputy constable said after he was captured, the shooter told him he didn't want to die.
"We have to relive it, not because we want to, but because we have to. If we didn't, where would we be right now? We'd be sitting at home with no answers, twiddling our thumbs. We refuse those kind of answers," Rosie Yanas, whose son, Chris Stone, was killed, said.
"If we can make a difference, it is worth reliving," Flo Rice, a substitute teacher who was shot, added. "It is worth reliving for one set of parents to lock those guns up and get their child mental help."
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