The designation "dangerous offender" is reserved for Canada's most violent criminals and sexual predators.

Crown attorneys can seek the designation during sentencing and must show that there is a high risk that the criminal will commit violent or sexual offences in the future.

The designation carries an automatic sentence of imprisonment for an indeterminate period, with no chance of parole for seven years.

Canada's dangerous offender laws have their roots in the 1947 Habitual Offender Act, which dealt solely with offenders with lengthy criminal records, and the 1948 Criminal Sexual Psychopath Act.

In 1977, the designation "dangerous offender" replaced both "habitual offender" and "dangerous sexual offender."

Johnson Aziga was convicted in 2009 of first-degree murder in the deaths of two of his sex partners through HIV transmission On Aug. 2, 2011, he was declared a dangerous offender. CBCChanges to the Criminal Code of Canada in 2008 require some repeat offenders convicted three or more times of violent crimes or sex crimes to prove that they are not a danger to society.

Putting the onus on the offender rather than on the Crown makes it easier to designate some repeat offenders as dangerous offenders, which effectively can put them behind bars for life.

Dangerous offenders can apply for parole after seven years, but the indeterminate sentence usually equals a life sentence.

Recently, Crown prosecutors have unsuccessfully sought the dangerous offender designation for serial drunk drivers.

Who are dangerous offenders?

According to the Correctional Service of Canada, there were 458 dangerous offenders as of April 2011.

Only a small number of people are designated dangerous offenders each year, but the number is climbing. In the 1980s about nine people per year were designated dangerous offenders. That number nearly doubled to 17 in the 1990s, and in the next decade, the number jumped to 25 per year.

In the year ending in April, another 26 people were labelled as dangerous offenders by the courts.

Renée Acoby is the only woman in Canada designated a dangerous offender. The Ontario attorney general requested Acoby receive the designation, after several violent incidents in prisons where she was incarcerated. A judge granted the request in March.

Before Acoby, only two women have ever been so designated. Marlene Moore committed suicide in prison in 1988. Lisa Neve had her designation lifted in 1999 and was freed after serving her sentence.

Aboriginal people account for 26 per cent of dangerous offenders but just four per cent of Canada's population.

Thomas Svekla, seen in the back of an RCMP cruiser after a 2006 court appearance, was declared a dangerous offender in May 2010. (CBC)A 2002 Correctional Service of Canada profile of 179 dangerous offenders shows that 85 per cent had committed sex offences, while only three per cent were convicted of a homicide.

Although the dangerous offender provisions can apply to all criminals, most people who are given such designations are in custody for sexual offences. For example, 85 per cent of dangerous offenders were sentenced for sexual assaults and 41 per cent involved pedophilia.

In addition to the dangerous offenders, as of April 2010, there were still 38 dangerous sexual offenders and nine habitual offenders in the system, who were categorized under those designations before the creation of the dangerous offender label in 1977.

Col. Williams case

Col. Russell Williams was convicted in October 2010 of 88 charges, including two counts of first-degree murder, which carry a mandatory life sentence without possibility of parole for 25 years.

The Crown attorney in that case said he would not seek dangerous offender status for Williams because it would be redundant.

"We built the record so the parole board will be satisfied that he will pose a danger for the rest of his life. A dangerous offender request is superfluous," said Lee Burgess.

However, murderer Paul Bernardo was given the designation in 1995 after being sentenced to life in prison without possibility of parole for 25 years.

The Crown sought the designation rather than hold trials on the outstanding charges Bernardo faced: 32 charges related to a series of rapes in east-end Toronto and a manslaughter charge in the death of Tammy Homolka, his sister-in-law.

Too easy for court to apply 'dangerous' label

Toronto's Alan Young, a criminal law professor at York University's Osgoode Hall Law School, told CBC News about two problems with the reverse-onus system.

The first, he says, is that it carries the risk of capturing the wrong people. The second, and more dangerous one, is that putting the onus on criminals makes it too easy for the court system to declare people dangerous offenders.

"It's almost imposing an impossibility, because you're saying to someone: 'Prove you're not dangerous,'" Young said, adding that the court system should make sentencing "more of an obstacle course, rather than an assembly line."

Regina defence lawyer Jeff Deagle has also criticized the application of the designation, especially in Saskatchewan.

"It is, essentially, a version of a three-strikes law," Deagle said, referring to the mandatory sentencing laws in place in 24 U.S. states.

"It's an easy solution, but it's not the right solution," Deagle told CBC News.

Saskatchewan accounts for 10 per cent of the more than 400 dangerous offenders with indeterminate sentences, even though the province has only three per cent of the country's population.