A librarian's quick thinking saved 50 students during the Florida school shooting.

When the words “Code red lockdown” crackled over her radio, Florida librarian Diana Haneski’s mind began to race.

She and her colleagues at the Marjory Stoneman Douglas High School had only recently had active-shooter response training, and were told to expect a drill.

But straight away, before a single gunshot had even echoed across the campus, she knew the threat was real.

In those moments, thoughts of a friend and fellow librarian came into Haneski’s head – Yvonne Cech of Sandy Hook Elementary. The 58-year-old has been credited with saving the lives of 22 people, including 18 students, by locking them inside a small supply closet during the horrific 2012 massacre.

The 20-year-old gunman took 28 lives that December morning, including his own. And had it not been for the quick thinking of Cech, it would likely have been more.

“She was there that day in Sandy Hook,” Haneski told Reuters, “and because of her I knew what to do.”

Speaking to the outlet in the wake of the deadly Valentine's Day shooting, the 57-year-old said she moved quickly to direct 50 students and five adults into a media equipment room at the rear of the library, shouting, "Get back in here! Get back in here!”.

Once inside, she locked the door and told everyone to sit down and hide behind something - boxes, stacks of paper, anything - as she closed the blinds and turned off the lights.

“I could see between the door and the floor. I could see if the bad guy - the shooter - walked past,” she said. “At first it was just lockdown, then right away we heard ‘shooting’. We heard his name.”

Nikolas Cruz, 19, stands accused of 17 counts of premeditated murder in what proved to be the second-deadliest school shooting in United States history after Sandy Hook.

Listen: Amelia Lester explains why US gun laws will never change, no matter how many lives are lost. (Post continues below.)

The Florida teenager, who had previously been expelled from the school for undisclosed disciplinary reasons, made a brief court appearance on Thursday where he was ordered to be held without bond. It's expected prosecutors will seek the death penalty.

"He's a broken human being," Cruz's public defender, Melissa McNeill, told reporters. "He's sad, he's mournful, he's remorseful."

As the first of many funerals for the victims were held on Friday, attention turned to the FBI amid revelations they failed to act on a tip received last months that Cruz had a "desire to kill", access to weapons and could be planning an attack.

But if you ask Haneski, it's gun laws that deserve scrutiny. Laws that mean a 19-year-old boy was able to get his hands on an assault rifle. Because despite the deaths of 20 first-graders six years ago in Sandy Hook, little has changed.

“People said to me: ‘What an incredible coincidence that you have such a close friend who could have the same experience’. I feel angry that anyone had to experience that horror," she told Reuters.

“We did everything we were trained to do. And still this happened. And still 17 didn’t go home after school.”