A former sheriff’s deputy has gone on trial charged with failing to protect students when a gunman opened fire in 2018 at a school in Parkland, Florida.
Scot Peterson was the on-duty police officer during the attack at Marjory Stoneman Douglas High School.
He is the first US officer charged with failing to respond to a school shooting, says the National Association of School Resource Officers.
The 60-year-old could receive up to 97 years in prison.
He faces 11 charges: seven of felony child neglect, three of culpable negligence and one of perjury.
Prosecutors say Mr Peterson, who was armed with a handgun but was not wearing a bullet-resistant vest, failed to follow his active-shooter training to minimise casualties during the six-minute attack.
He did not enter the building as shots were fired, according to video footage.
Seventeen people died and 17 others were injured in what remains the deadliest school shooting in US history.
To convict Mr Peterson, prosecutors must persuade the jury he knew the gunman was inside the building and that his alleged inaction endangered students. They must also show he was legally a caregiver to the students.
Mr Peterson has maintained he thought the shots were coming from a sniper outside, otherwise he has insisted he would have gone inside the building to confront the attacker.
Defence attorneys have said they will argue their client does not fall under the statutory definition of “caregiver”. The neglect law is typically used to prosecute parents or daycare providers.
The misdemeanour perjury count Mr Peterson faces is for allegedly lying to investigators under oath about his actions that day.
David Weinstein, a Miami criminal defence lawyer and former prosecutor, told BBC the prosecution will have an “uphill battle” proving that Mr Peterson had the legal responsibility to be the caregiver of the children in this case.
“Just because somebody’s job description contains the words ‘school resource officer’ doesn’t mean he’s defined under the statute as a caretaker of these children,” Mr Weinstein said.
“Traditional law enforcement do not have that legal responsibility over a child.”
This video can not be played
To play this video you need to enable JavaScript in your browser.
In a statement on Tuesday, his attorney Mark Eiglarsh said Mr Peterson “did all he could to save lives”.
“We have overwhelming evidence proving that my client didn’t know precisely where the shooter was during the attack and also, of the numerous actions that my client took during the attack to save lives,” the lawyer said.
Mr Peterson, who retired after 32 years as a deputy in the aftermath of the attack, was charged in June 2019.
If convicted, he also faces losing his $104,000 (£83,500) annual pension.
A Florida Department of Law Enforcement investigation found Mr Peterson “did absolutely nothing to mitigate” the shooting.
Their inquiry found the gunman discharged his weapon about 140 times, 75 times after Mr Peterson arrived on the scene.
Critics, including then-President Donald Trump, branded him a coward.
The attacker pleaded guilty to the killings and was sentenced last year to life in prison without parole.