O'Rourke is charged with seven counts of attempted murder and assault with a firearm. He has pleaded not guilty by reason of insanity.
When opening statements began shortly before 9:30 a.m., prosecutors painted a vivid picture of what happened that afternoon on October 8, 2010 calling the shooting a terrorist attack.
They described the weapon O'Rourke was carrying as an especially lethal revolver. In pretrial hearings, witnesses testified O'Rourke jumped a fence carrying a .357 magnum with pockets full of ammunition and a gas can.
Prosecutors say O'Rourke went to Kelly Elementary that day with the intent to kill as many children as he could.
That is why, they say, he targeted the school at 1210 in the afternoon, when most children would be outside on the playground.
"He would point, aim at the level of the children...would fire into a crowd of 15 to 20 kids, then he would pivot his hand and fire again and shoot into a group of 20 to 25 of children a few feet away from him," said Deputy District Attorney Summer Stephan.
Defense Daniel Segura asked the jurors to keep an open mind.
The first witness will be a 7-year-old girl who was shot in the arm but the judge ordered no cameras be allowed in the courtroom during this time.
Two girls who were shot in the ambush each took the stand Monday our media partner the North County Times reported.
"I saw a man on our field who did not look familiar to me, and he had a gun," said one.
"I started to run, and he shot me," testified the other girl who was shot in the attack.
O'Rourke faces 103 years in prison if convicted of all charges.
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