Mexican authorities say they apprehended Amelie Pelletier and Fallon Rouiller Poisson earlier this week after a government office in the capital and an adjacent car dealership were struck by Molotov cocktails.
Mexico's federal attorney-general's office told The Canadian Press it viewed the firebombing as vandalism, but the ongoing investigation could lead to more serious criminal charges.
Spokesman Jose Luis Manjarrez says investigators were trying to determine what kind of role the Canadians may have played in an attack that damaged several vehicles at the car dealership. No injuries were reported.
"For now, for the authorities, it is considered an act of vandalism and not a terrorist act," Manjarrez said Thursday in a phone interview.
"It's an act of provocation by people who are demonstrating their disagreements by using radical means.
"But to be considered a terrorist act, it must correspond to a precise legal definition and that will be up to a judge to determine after the evidence has been examined."
The motive was not immediately clear, officials said.
A Mexico City prosecutor has told local media that police allege one of the Canadians was spotted discarding a backpack that contained a number of homemade explosive devices.
Police also arrested a Mexican national following the Molotov-cocktail attack, which hit the facade of a building that houses the offices of the Department of Communications and Transportation.
Manjarrez said investigators were trying to determine whether the intended firebomb target was the government office or the car dealership.
The fate of the women has raised concerns back in Canada.
On Thursday in Montreal, dozens of people showed their support for Pelletier and Poisson by holding a vigil outside the front entrance of a downtown building where the Mexican Consulate has an office.
Event organizers declined to answer questions from reporters, but a statement was read aloud to the gathering by well-known activist Jaggi Singh.
The document was signed by a group that calls itself the Committee of Support for Imprisoned Comrades.
"The Mexican justice system, like all systems created to protect private property and the interests of the rich, is repressive and authoritarian and considers those detained to be guilty until proven innocent," Singh said as he read the statement to some 40 or 50 supporters.
Those who know Poisson, a recent junior-college graduate, have said she was an active participant in the 2012 student movement that led to nightly demonstrations in Quebec — a crisis dubbed the Maple Spring.
She was also described as someone with strong political convictions.
In the support group's statement on Thursday, vigil organizers expressed concern Poisson and Pelletier could become victims of "political profiling."
"The identities and affiliations of the accused can become 'evidence' used to punish the accused far beyond the scope of the original crimes being investigated," the message read.
Participants then chanted, "Free our comrades," as they huddled on a sidewalk under the watchful eyes of nearby police officers.
They also unfurled a banner with a message, partially Spanish and partially French: "Not guilty, Not innocent, SOLIDARITY."
Poisson's family has also expressed concern following her arrest in Mexico City.
Her mother, Line Rouiller, told The Canadian Press on Wednesday she feared for the well-being of her 20-year-old daughter, who grew up in the Montreal area.
Rouiller said while her daughter was someone who had taken part in protests, she was not the type of person who would damage property.
Canada's Foreign Affairs Department, meanwhile, confirmed its officials have been in contact with Mexican authorities on behalf of two detained Canadian citizens. The department, however, said it could not release any more information about them.
"What I can say is consular services are being provided as required," spokesman John Babcock wrote Thursday in an email.
"To protect the privacy of the individuals concerned, further details on this case cannot be released."