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 has said.
Deputy Scot Peterson resigned after an investigation found he failed to confront the suspect.
Trump on Friday said Peterson might be a "coward" who "didn't react properly under pressure", BBC reported.
Florida's governor meanwhile called for law enforcement officers to be placed in every public school in the state.
Speaking to reporters outside the White House, President Trump criticised the police officer who did not confront the perpetrator of the February 14 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.
Broward County Sheriff Scott Israel revealed on Thursday that 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 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 Peterson should have done, Sheriff Israel said: "Went in, addressed the killer, killed the killer."
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.
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.
Peterson had been in his position at the school since 2009.