Vince Li was declared not criminally responsible last year for the killing of Tim McLean on a Greyhound bus in 2008. (John Woods/Canadian Press)

The Crown attorney's office in Manitoba has decided not to appeal a decision that granted Greyhound bus killer Vince Li his first small taste of freedom.

The Crown says there are no legal grounds to challenge a ruling by a criminal review board to allow Li short, supervised walks on the grounds of the mental hospital where he is being held.

Li was found not criminally responsible for stabbing and beheading Tim McLean on a Greyhound bus in July 2008.

He has been held in a locked section of the Selkirk Mental Hospital north of Winnipeg.

The Criminal Code Review Board, which examines Li's case every year, decided in the spring that he should be allowed to walk outside, as long as he is with two staff members at all times. It would start at 15 minutes a day and work up gradually to two hours a day.

The decision was a blow to the victim's family, who want Li locked up for the rest of his life.

Manitoba Attorney General Andrew Swan has said Li's walks will not start until the mental hospital increases security measures.