The guards pushed Bertrand into an adjacent room, where the action continued out of shot, but his mother Julie Bertrand watched on in horror.
Suddenly she began screaming and Lacaille ran round to the entrance of the other room in time to see a constable shoot Bertrand.
Julie and Lacaille began screamingly and crying hysterically as their relative lay bleeding on the ground.
"It's not an experience to live. I can not stop thinking about that" Lacaille told Le Soleil.
Bureau of Independent Investigation, EIB, officers, who are investigating the shooting, say that the 18-year-old had managed to seize the special constable's telescopic stick and hit him on the head.
The constable had then taken out his gun, and shot the teen in the head.
Bertrand was taken to hospital where he is reportedly in a stable condition.
Incredibly, his family say the bullet narrowly missed his eye and brain and he is expected to recover.
"Steven is fine," Lacaille said, in French, on Facebook. "He was touched close to the eye. The bullet has ricocheted up to the neck, but it is fine. He will get by. No problem to the brain, everything is beautiful! Thank god."
The constable who was hit in the head with the baton is also in a stable condition, according to the EIB.
The footage of the shooting rapidly went viral after Lacaille posted it on Facebook yesterday, with more than 1.4 million times, 36,000 shares and thousands of comments.
The EIB said the reaction to the video has been so strong, that several viewers had made threats to the special constable after watching it.
On Thursday morning, the Minister of Justice Stéphanie Vallée, called for restraint.
"Threats, words of this nature can lead to accusations. We must be reserved not only in what we say in everyday life, but also in our comments on social networks.
"We can be shocked by the images we see. One can be shocked by a situation, but to answer that way, to respond with violence, no matter in what situation, it can have a boomerang effect. I find it totally unacceptable to make such comments on social media."
It's not yet clear what new charges Betrand will face when he recovers from his gunshot.