The Canadian Broadcasting Corporation describes it as "the principal base for live fire training exercises". About 690 military personnel and 300 civilian workers are on site on a regular day, according to the CBC.
Saturday's blaze "spread rapidly due to high winds" and had torn through the fences of the base, the Municipal District of Wainwright said.
McGuinty told reporters he "cannot recall an incident like this in recent memory".
Alberta's premier Danielle Smith says the "significant and tragic fire" is "affecting many of [the province's] brave military personnel". Provincial authorities are in touch with the central government in managing and investigating the situation, she added.
It is unclear when the fire broke out, but the district first posted about the incident at about 17:30 local time (23:30 GMT).
Local authorities have urged residents to avoid the area and give all emergency vehicles right of way.
Neighbouring provinces have offered their support.
Saskatchewan premier Scott Moe says he has deployed resources to Alberta and is "ready to provide more if required", while British Columbia premier David Eby says his government has "reached out to Alberta to offer all possible assistance".
Extracted and lightly reformatted for readability. · Source: pt