(NewsNation) — Video showed smoke and people pouring out of a New York subway car in the aftermath of what police say was a shooting that left 17 injured Tuesday morning.

New York Police Commissioner Keechant Sewell said the suspected gunman donned what appeared to be a gas mask. He then took a canister out of his bag and opened it. The train then began to fill with smoke.

Investigators are examining whether the suspect deployed the smoke device in an effort to distract people before opening fire, a law enforcement official said.

Jeff James, a retired secret service agent of more than two decades, told NewsNation it’s a tactic police train for.

“We practice scenarios where we put on our gas mask and we get into the fight,” James said. “It does affect your breathing, it affects your vision, it affects your field of vision, as well as your clarity of vision. So yeah, this isn’t something that especially the NYPD, which is a premier law enforcement agency in the world, this isn’t something that if they had to, to send people down into that tunnel, it isn’t something that they wouldn’t have drilled at some point.”

James and other law enforcement sources said smoke canisters — which can be bought online for as little as $12 — are most often used in military operations.

Smoke canisters are also commonly used by law enforcement to help encourage peaceful surrenders.

But this isn’t the first time smoke has been used in a shooting.

The man who killed five Capital Gazette newsroom employees reportedly set off a smoke grenade in a hallway before shooting and killing five journalists in the Maryland newsroom in 2018.

Parkland shooter Nikolas Cruz was armed with smoke grenades, too, when he shot 34 people with an AR-15 at Marjory Stoneman Douglas High School in 2018.

“I’m not shocked that a tactic like this was used as a diversion and also to create fear,” James said.

Law enforcement said after situations like this, there is always a concern about copycats.

According to James, that is another reason it’s essential to be fully aware of your surroundings and not be totally distracted by headphones or mobile devices — especially while on public transit.