OTTAWA — Intelligence officials are concerned that extremists might infiltrate peaceful anti-petroleum protests to "incite violence," a newly disclosed assessment indicates.

The federal analysis of threats to the passenger rail system introduces a new twist to the often tense debate over state scrutiny of environmental demonstrators — that otherwise harmless activists might unwittingly harbour dangerous terrorists.

The assertion — contained in a Transport Canada intelligence report — led one defender of civil liberties to ask whether there is evidence to support it.

"I do think it's dangerous to start painting activism as a potential cover for terrorist activities," said Cara Zwibel, director of the fundamental freedoms program at the Canadian Civil Liberties Association.

The Canadian Press recently obtained a copy of the six-page, November 2014 analysis by Transport's security intelligence assessment branch through the Access to Information Act.

Overall, the assessment warns that surface transportation such as passenger rail is "a favoured target of terrorists" because attackers can inflict mass casualties, fear and economic harm using simple tactics — particularly improvised explosive devices.

It points to the 2004 Madrid and 2005 London bombings as well as a more recent, failed plot to derail a Via Rail passenger train — all led or supported by al-Qaida.

Future plots against passenger rail in Canada are possible given the emphasis on "solo jihad" by al-Qaida and the Islamic State of Iraq and the Levant, past statements from their members and supporters, and the ease with which an attack could be carried out, the report says.

But it also cites the prospect of violence by so-called domestic extremists — people on the left or right end of the spectrum motivated by various political, social, environmental and aboriginal issues. Possible attacks or sabotage against rail transportation "are of concern," the assessment says.

At the same time, there are many "legitimate, non-violent activist groups" in Canada that seek to influence policy and garner publicity through protests — sometimes by blocking rail lines, the report notes.

"Currently in Canada, there is considerable opposition to pipeline construction, oil sand extraction and the movement of crude oil by rail. Protests and demonstrations at or near surface infrastructure are ongoing and may increase in the near-term," the assessment adds.

"While these types of events are not inherently a threat to security, domestic extremists may seek to infiltrate these events and use them as opportunities to incite violence. A vigorous protest cycle may trigger an increase in domestic extremist activity."

After reading the intelligence assessment, Zwibel is curious about the basis for this assertion.

"I'm just wondering where that comes from — if there's some good evidence to suggest that that's the case, it's not in here as far as I can tell," she said.

Zwibel is concerned that authorities could use the notion of infiltration as justification "to engage in surveillance or profiling of legitimate, non-violent activist groups."

The Conservative government's omnibus security bill, which received royal assent last June, drew fierce criticism from environmentalists, aboriginal leaders and others who feared the provisions could be used to spy on dissenters — something the government denied.

The new Liberal government has promised to repeal "problematic elements" of the legislation but it is unclear how extensive the revamp might be.

Jim Bronskill, The Canadian Press