The armed officer on duty at the Florida school where a shooter killed 17 people never went inside to engage the gunman and has been placed under investigation, officials have announced.

The school resource officer at the high school took up a position viewing the western entrance of the building that was under attack for more than four minutes, but "he never went in," Broward County Sheriff Scott Israel said at a Thursday news conference. The shooting lasted about six minutes.

The officer, Scot Peterson, was suspended without pay and placed under investigation, then chose to resign, Israel said.

When asked what Peterson should have done, Israel said the deputy should have "went in, addressed the killer, killed the killer."

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The sheriff said he was "devastated, sick to my stomach. There are no words. I mean these families lost their children. I've been to the funerals. I've been to the vigils. There are no words."

Law enforcement officers block off the entrance to Marjory Stoneman Douglas High School on the day of the shooting, which left 17 people dead. Photo / AP
Law enforcement officers block off the entrance to Marjory Stoneman Douglas High School on the day of the shooting, which left 17 people dead. Photo / AP

Coral Springs Officer Tim Burton revealed Peterson hid from Cruz when the teenager started shooting, the Daily Mail reported.

Burton said Peterson "was seeking cover behind a concrete column leading to a stairwell", because he was worried Cruz could be lurking in the lot. He said Peterson couldn't hear gunshots or screams to lead him to the precise location of the shooting.

The Valentine's Day shooting at Marjory Stoneman Douglas High School by a gunman armed with an AR-15 style assault rifle has reignited national debate over gun laws and school safety, including proposals by President Donald Trump and others to designate more people — including trained teachers — to carry arms on school grounds. Gun-control advocates, meanwhile, have redoubled their push to ban assault rifles.

There was also a communication issue between the person reviewing the school's security system footage and officers who responded to the school.

Coral Springs Police Chief Tony Pustizzi said during a Thursday news conference that the footage being reviewed was 20 minutes old, so the responding officers were hearing that the shooter was in a certain place while officers already in that location were saying that wasn't the case.

"There was nothing wrong with their equipment. Their equipment works," Pustizzi said.

"It's just that when the person was reviewing the tape from 20 minutes earlier, somehow that wasn't communicated to the officers that it was a 20-minute delay."

Scot Peterson - the armed security guard who allegedly sought cover while the gunman shot schoolchildren - is seen speaking at a February 2015 school board meeting. Photo / Supplied
Scot Peterson - the armed security guard who allegedly sought cover while the gunman shot schoolchildren - is seen speaking at a February 2015 school board meeting. Photo / Supplied

Pustizzi said the confusion didn't put anyone in danger.

The shooting suspect, 19-year-old Nikolas Cruz, has been jailed on 17 counts of murder and has admitted the attack. He owned a collection of weapons.

Defence attorneys, state records and people who knew him indicate that he displayed behavioral troubles for years.

Broward County Sheriff Scott Israel speaks before a CNN town hall broadcast this week. Photo / AP
Broward County Sheriff Scott Israel speaks before a CNN town hall broadcast this week. Photo / AP

Broward County incident reports show that unidentified callers contacted authorities with concerns about Cruz in February 2016 and November 2017.

The first caller said they had third-hand information that Cruz planned to shoot up the school. The information was forwarded to the Stoneman Douglas resource officer.

The second caller said Cruz was collecting guns and knives and believed "he could be a school shooter in the making".

Politicians under pressure to tighten gun laws in response to the mass shooting floated various plans Thursday, but most fell short of reforms demanded by student activists who converged Wednesday on Florida's Capitol.

US Senator Marco Rubio, of Florida, said a visit to Stoneman Douglas prompted him to change his stance on large capacity magazines. The Republican insisted he is willing to rethink his past opposition on gun proposals if there is information the policies would prevent mass shootings.

"If we are going to infringe on the Second Amendment, it has to be a policy that will work," Rubio said in an interview Thursday with The Associated Press.

State Senator Bill Galvano, who is helping craft a bill in response to the shooting deaths, said an idea gaining traction is a program that would allow local sheriffs to deputize someone at a school to carry a gun on campus.

Galvano insisted the idea is not the same as arming teachers. He said the program would be optional and the deputised person would have to be trained by local law-enforcement agencies.

Florida Senate President Joe Negron said both chambers are working on the legislation in response to the Parkland shootings. He said a final draft should be available "early next week at the latest."

Republican legislative leaders in Florida say they will consider legislation that will likely call for raising the age limit to purchase a rifle from 18 to 21 and increasing funding for mental health programs and school resource officers, the police assigned to specific schools. Legislators may also enact a waiting period for rifle purchases. What won't be considered is a ban on assault-style rifles.

A day after an emotional meeting with survivors and their families, Trump tweeted his strongest stance yet on gun control.

He said he would endorse strengthening background checks, banning "bump stock" style devices and raising the minimum age to 21 for buying certain rifles.

At a conference of conservative activists Thursday near Washington, Vice President Mike Pence said the administration would make school safety "our top national priority" after the shooting at the school in Parkland, Florida.

Calling school shootings "evil in our time," Pence exhorted those in positions of authority "to find a way to come together with American solutions."

It was a markedly different tone than that deployed on stage minutes earlier by NRA Executive Vice President and CEO Wayne LaPierre, who delivered an unbowed defence of gun ownership and lashed out at Democrats — saying they are using the tragedy for "political gain".

On Thursday, Democratic Senator Bill Nelson gave Rubio credit for being the only Republican to attend a televised town hall Wednesday night held in the aftermath of the school shooting and criticized Republican Gov. Rick Scott for not showing up.

"I commended (Rubio) for being there. He had the guts to be there when Governor Scott did not," Nelson told a group of Democratic state senators.

Scott is likely to challenge Nelson as he seeks a fourth term in the Senate this November. Nelson questioned Scott's commitment to make meaningful change after the shooting.