OTTAWA - The sabotage of a CN Rail crossing signal during an aboriginal protest and rail line blockade near Belleville, ON over the weekend should be seen as an act of terror, says one security expert.

"This is a low-level, low-grade form of terrorism," said John Thompson, with the Mackenzie Institute think-tank. "It's not as dramatic as car bombings and so on and so forth, but ... the use of vandalism is a form of violence."

CN Police says it's investigating after a group of Idle No More protesters allegedly activated a crossing signal Saturday afternoon, and lit a fire on the tracks.

The blockade within Mohawk territory also grabbed the attention of Ontario Superior Court Judge David Brown.

He issued an injunction against the blockade on Saturday, but said Monday he's mystified by the refusal of Ontario Provincial Police to enforce it before protesters dispersed.

Brown also questions the judgement of an officer who said in a sworn statement that it was "too dangerous" to enforce the order on 15 protesters.

"That kind of passivity by the police leads me to doubt that a future exists in this province for the use of court injunctions in cases of public demonstrations," said Brown.

Thompson says infrastructure such as railways and pipelines remain vulnerable to a "tiny militant minority" of aboriginal protesters because of their rural location.

"We can't secure our infrastructure and they've known for a long time that they can tamper with things as they choose," he said.