1988 attack killed 259 people in the air and 11 on the ground
RCMP provided John Nuttall and Amanda Korody with explosives and fake detonators, then arrested them
Men were at remembrance rally for the 1984 massacre of Sikhs in India
Police say device was strategically placed along the half-marathon path to cause panic
The 49-year-old plead guilty to possessing or making explosives and mischief
A warrant has been issued for Blair Robert Balch
RCMP say a criminal investigation into the matter is still ongoing
Police and canine units are surrounding Royal Inland Hospital