Trump visits Florida massacre survivors as FBI admits it mishandled tip

US President Donald Trump on Friday visited a Florida hospital to offer comfort to those wounded in a mass school shooting, after the FBI admitted it mishandled a tip about the teenager behind the massacre that left 17 dead. 

The arrival of Trump and his wife Melania came at the end of a difficult day for the families of those killed in Wednesday's rampage at a high school in Parkland, Florida, who learned that the carnage could perhaps have been averted.

The FBI, the top US law enforcement agency, admitted it had received a chilling warning in January from a tipster who said the 19-year-old attacker, Nikolas Cruz, could be planning a mass murder, but that agents failed to follow up.

At Broward Health North Hospital, where Trump met with survivors of the shooting, the president thanked the doctors, nurses and first responders for their "incredible" work, and described the carnage as "very sad."

Steve Mort reports.

The president's visit to Parkland - an area about 80 kilometres north of Miami - came amid growing anger among parents and students in the south Florida city over America's seeming unwillingness to toughen gun control laws.

"My princess wasn't safe in that school," said Andrew Pollack, speaking at the funeral of his 18-year-old daughter Meadow. "Please pray that this horrific tragedy never happens to another family."

Admitting error 'isn't going to cut it'

The FBI made a stunning admission earlier in the day, saying a "person close to Nikolas Cruz" made a call to the agency's public tipline on January 5 to "report concerns about him."

"The caller provided information about Cruz's gun ownership, desire to kill people, erratic behaviour, and disturbing social media posts, as well as the potential of him conducting a school shooting," the FBI said in a statement.

The information from the caller "should have been assessed as a potential threat to life" and forwarded to the agency's Miami field office, it said.

Instead, "no further investigation was conducted."

"We have spoken with victims and families, and deeply regret the additional pain this causes all those affected by this horrific tragedy," said Federal Bureau of Investigation director Christopher Wray, who took up his post in August.

Wray said he was "committed to getting to the bottom of what happened," and Attorney General Jeff Sessions ordered an immediate review to ensure "effective response to indications of potential violence."

But Florida Governor Rick Scott nevertheless called for the FBI chief to step down, saying the failure to act was "unacceptable."

"Seventeen innocent people are dead and acknowledging a mistake isn't going to cut it," Scott said.

"I'm going to be a professional school shooter"

The January warning to the FBI was not the first it had received about Cruz, who used an AR-15-style semi-automatic rifle that he legally purchased a year ago to spray classrooms with bullets.

In September, the FBI was alerted to a message posted on YouTube, in which a user named Nikolas Cruz vowed: "I'm going to be a professional school shooter."

The FBI said it had looked into it at the time but was unable to identify the person who made the post. 

Trump's visit to the Parkland area - not far from his Mar-a-Lago resort, where he was spending the long President's Day weekend - was not announced in advance, perhaps because he risked being greeted with angry demands for action on laws that allowed Cruz to amass an arsenal.

"It's illogical that the law says a minor can't have a drink, but can buy a gun," said 47-year-old Mavy Rubiano, whose child survived the shooting.

Focus on gun control

In Washington, the political response so far makes it clear that the powerful pro-gun National Rifle Association - which spent $30 million to support Trump's election in 2016 - remains formidable.

On Thursday, Trump's nationally televised address made no mention of guns, or of previous mass shootings.

The president focused instead on offering sympathy to the families of the victims and the need to provide better mental health care.

As with previous mass shootings, the focus of gun control advocates was the easy availability of the AR-15, a civilian version of the US military's M16.

Millions have been sold around the United States, and AR-15-style rifles were used in the mass shootings in Las Vegas, Sutherland Springs, Texas and Newtown, Connecticut.