An armed officer who stood outside a Florida school where a gunman killed 17 people last week "certainly did a poor job", US President Donald Trump says.
Deputy Scot Peterson resigned after an investigation found he failed to confront the suspect.
Mr Trump said Mr Peterson might be a "coward" who "didn't react properly under pressure".
Florida's governor meanwhile called for law enforcement officers to be placed in every public school in the state.
What did Trump say?
Speaking to reporters outside the White House on Friday morning, President Trump criticised the police officer who did not confront the perpetrator of the 14 February massacre in Parkland, Florida.
"He trained his whole life but when it came time to do something he didn't have the courage," he said.
"He certainly did a poor job."
"But that's a case where somebody was outside, they're trained, they didn't react properly under pressure or they were coward," he added.
The Republican president later touted his love for gun rights on Friday at the Conservative Political Action Conference (Cpac) in Maryland.
He again proposed arming teachers as a solution to school safety, a method long championed by the National Rifle Association (NRA) gun lobby.
What did the officer do?
Broward County Sheriff Scott Israel revealed on Thursday that Mr Peterson, the 54-year-old school resource officer, stood outside while the alleged gunman shot students at Marjory Stoneman Douglas High School.
He said video footage showed Mr Peterson arriving at the building where the shooting broke out about 90 seconds after the first shots were fired, and that he remained outside for about four minutes. The attack lasted six minutes, Sheriff Israel said.
"I am devastated. Sick to my stomach. He never went in," Sheriff Israel said.
Asked what Mr Peterson should have done, Sheriff Israel said: "Went in, addressed the killer, killed the killer."
Mr Peterson is yet to publicly comment on what happened. Officers are reportedly guarding his home. It is unclear if he will face charges.
The suspect used a semi-automatic AR-15 rifle, police say, and escaped the scene before being captured later.
What was Deputy Peterson's role?
School resource officers are sworn law enforcement officers who are responsible for safety and crime prevention in schools, although their exact roles differ from school to school and authority to authority.
Employed by the local police or sheriff's office, they document incidents and can make arrests, as well as working on areas such as mentoring and education.
There are between 14,000 and 20,000 such officers in the US, according to the National Association of School Resource Officers.
Mr Peterson had been in his position at the school since 2009, local media report.
The Sun-Sentinel newspaper said he had worked for the sheriff's office since 1985 and office records showed his salary for 2016 was $75,673 (£54,137).
Were there any other failures?
Concerns have been raised about both the school's video surveillance system and whether warnings about the former student charged with the massacre were ignored.
It has been reported that the person watching the surveillance system was relaying information 20 minutes old to police, so officers believed the gunman was in a certain area when he was not.
Calls were also reportedly made to the authorities in 2016 and 2017 expressing concern about the suspect, including one saying he was planning to attack the school.
A transcript of one call to the FBI was released to US media on Friday, in which a tipster warns that Mr Cruz "wants to kill people" and that "he's going to explode".
The female caller, whose identification was redacted in the report, said that Mr Cruz once brought a bird inside "threw it on his mother's kitchen counter and he started cutting it up".
She also warned that Mr Cruz was receiving an inheritance of $25,000 per year, and that he may spend that money on guns.
What is Florida's governor proposing?
Governor Rick Scott called for "active shooter training" for all students and staff at public schools in the state.
He said he planned to raise Florida's minimum age for buying guns to 21 years old.
Governor Scott also pledged to make it "virtually impossible for anyone who has mental health issues to use guns."
"If a court involuntarily commits someone because they are a risk to themselves or others, they would be required to surrender all firearms and not regain the right to purchase or possess a firearm until a court hearing," he said during a speech about a new proposal on guns in Tallahassee.
Mr Trump added: "What I'd recommend doing is the people that do carry, we give them a bonus. We give them a little bit of a bonus."
But Randi Weingarten, president of the 1.7 million-member American Federation of Teachers union, disagreed that arming teachers was a suitable solution.
"Anyone who wants guns in schools has no understanding of what goes on inside them - or worse, doesn't care," she said.