This scene looks a response to a terrorist strike unfolding on the streets of Los Angeles.

But you are watching a simulation of how law enforcement would react if there was a real attack on American soil.

Cops on the front lines, the first defense against a strike, foreign or homegrown, on a never-ending training mission for an assault that could happen at any moment.

And on this night, Crime Watch Daily cameras are invited to follow an elite counterterrorism unit during their high-level training for such an event, a simulated military exercise that most people would never see, unfolding underground in Los Angeles.

Police have been told a group of terrorists has just launched a series of coordinated attacks targeting the L.A. subway system. We know that there are hostages, and there are reports of injuries, but most alarmingly, they have detected the presence of a device which appears to have dispersed some sort of chemical agent.

When carrying out a mission, every second counts. First, the unit is briefed on the threat they are about to face.

After getting suited up in state-of-the-art tactical gear, the team enters the subway.

"Tonight's device is a device that has been seen as an Al Qaeda- or an ISIS-type device that they've worked on in the past, and without getting into the specifics of it, that's what we're going to simulate tonight," says L.A. County Sheriff's Sgt. Jeffrey Ivask.

The team stalks the subway, looking for a suspect or a device that could disperse a potentially deadly chemical.

This unit's mission is very similar to the operation used to counteract the Tokyo sarin-gas attack of 1995, when the deadly nerve gas spewed from wrapped packages planted throughout the Japanese subway lines, injuring more than 5,000 and killing 13.

Once on the subway car, their suspect is identified and the unit closes in.

"Now that we've identified this, it's going to be a much larger-scale exercise than we had originally anticipated. They are giving him commands, he is not complying," said Sgt. Ivask. "We are going to hold this indefinitely until we can get people here to negotiate."

While the gunman is holding his hostage at gunpoint, the worst-case scenario erupts.

"Our suspect has set off the cyanide device," says Ivask. "You can see the gas moving across the floor here at this time. The victim is choking.

"Suspect's down, now they are going to try to effect an immediate rescue to get our victim out to emergency 'decon,'" says Ivask.

After getting their hands on the suspect and freeing the hostage, officers drag the two men out onto the platform.

"With tonight's scenario, this is going to simulate a release of a cyanide gas, being a blood agent, it would affect one's ability to breathe in it," said Ivask. "Cyanide could be a long painful death, if you will, protracted over several hours, but it's a matter of getting in there quickly and effecting rescues."

After the threat is contained, the unit heads up to street level for decontamination.

"So we are going to use a 'decon' solution when they first come up, that's what they're spraying on them, that's going to neutralize any agent they were exposed to," said Sgt. Jeff. "They'll get brushed off, so any gross contamination particles, that will be scrubbed off them as well."

Containing a chemical threat just scratches the surface of what this unit can do. These guys train for years preparing for other kinds of terrorist attacks, from radiological and nuclear to explosive and biological threats.

What this drill has really hammered home is that terrorism isn't just somebody else's problem. It's knocking at our door. And while these deputies firmly believe that it is a matter of when, and not if, there is at least some small comfort in knowing that when it comes to the Los Angeles County Sheriff's Department, there are few better agencies anywhere in the world.