Before Ontario Superior Court Justice David McCombs denied bail this week to a 16-year-old girl charged with first-degree murder, he, like any other judicial official deciding whether to release someone from custody, had three options to weigh.

Is the detention necessary to ensure the accused shows up for court?

Is the accused, if released, likely to commit a criminal offence that will endanger the public?

And are the facts and the nature of the crime itself so shocking to the community that the release of the accused would bring the administration of justice into disrepute?

Where a judge or justice of the peace is satisfied that one of the grounds is present, he or she may make an order under subsection 515(5) of the Criminal Code that a person be detained in custody.

The grounds are called primary, secondary and tertiary. McCombs used the tertiary ground to detain the teenager, whose identity is protected under the Youth Criminal Justice Act.

Her 18-year-old boyfriend is also charged with the New Year's Day fatal stabbing of 14-year-old Stefanie Rengel, the daughter of Toronto police officers.

The tertiary ground is supposed to be used "sparingly," say legal experts, and "mostly it has to do with public policy concerns," explained defence lawyer Sean Robichaud.

A judge or JP considers the strength of the Crown's case, the nature of the crime and what sentence could be imposed if there's a conviction.

In most bail hearings, the Crown must demonstrate why detention is necessary. In some circumstances, such as murder cases, the accused must demonstrate why his or her release is justified.

To be granted bail, a person is typically released into the care of a surety who pledges a certain amount of money.

The surety must be prepared to lose this – or part of it – if the person breaches any of the court's "conditions of release" or "recognizance" while on bail.

Robichaud said one of the biggest problems in the justice system is the bail process, particularly for people facing less serious charges. In provincially run facilities, more than 60 per cent of people in custody are awaiting trial.

"What that says is there are more presumed innocent people in jail than there are guilty people, which is just very odd."

James Stribopoulos, a law professor at Osgoode Hall, said there are "systemic factors" to explain why so many people remain in custody before due process takes its course.

"We don't live in a perfect world and our justice system is far from perfect, we have limited resources so ... once your case is ready you often have to wait," he said.

At a certain point though, charges can get thrown out due to unreasonable delay, he noted.

"Bail is the most important decision that gets made in the process, short of the determination of guilt or innocence at trial," said Stribopoulos.

"If someone is detained the impact on them can be pretty profound. If you're an adult, you could lose your job, lose your apartment. It's pretty hard to maintain a relationship or contact with relatives when you're in custody."