An armed police officer who stayed outside a Florida senior school during a mass shooting has been arrested on 11 criminal charges.

Former deputy Scot Peterson (56) is accused of child neglect, culpable negligence and perjury - charges that carry a combined potential prison sentence of nearly 100 years, prosecutors say.

He was on duty at Marjory Stoneman Douglas High in Parkland on Valentine's Day last year but never went inside while bullets were flying.

The officer was branded a "coward" by US President Donald Trump for failing to confront the gunman.

Sheriff Bob Gualtieri, chairman of the Marjory Stoneman Douglas High School public safety commission, said the charges against Peterson were "absolutely warranted".

"There is no doubt in my mind that because he didn't act, people were killed," Mr Gualtieri said.

Nikolas Cruz (20) faces the death penalty if convicted of killing 17 people and wounding 17 others in the attack.

His lawyers have said he would plead guilty in return for a life sentence, but prosecutors have refused that offer and he is expected to go on trial in early 2020.

Thousands attend an interfaith service at Pine Trails Park in Parkland, Florida to remember 17 victims killed at Marjory Stoneman Douglas High School | Image: Sun Sentinel/TNS/ABACA/ABACA/PA Images

Tony Montalto, whose 14-year-old daughter Gina Montalto was one of the victims, said families wanted justice to be done.

He said: "We are happy to see some accountability for this tragedy that took the life of my daughter Gina and 16 other wonderful individuals as well as terribly injured 17 others."

Peterson's bail has been set at US$102,000 (€90,496).

His lawyer did not immediately respond to a request for comment but has previously defended his client's conduct as justified under the circumstances.

Broward County Sheriff Gregory Tony said Peterson's employment has been formally terminated, although he announced his retirement shortly after the shooting.

Another deputy, former Sergeant Brian Miller, was also fired, although he faces no criminal charges for his actions that day.

Peterson's arrest is the latest fallout from the Parkland school shooting.

Former sheriff Scott Israel was suspended for "neglect of duty and incompetence" over the department's actions that day.

He is appealing that decision to the state Senate and said he intends to run again next year.