LITHIA, Florida -- "Police with a warrant! Open the door!" one agent shouts.

The next sounds you hear: a team of special agents forcing open a front door, and a flash bang tossed inside to distract the suspects as it explodes. Then, shots ring out as those agents hone in on their targets.

From drug smuggling to child exploitation, some of the most dangerous offenders and shocking cases are handled by special agents with Homeland Security Investigations (HSI), one of the largest components within U.S. Immigration and Customs Enforcement (ICE).

Training as a special agent with HSI is rigorous and can take months to get used to, with simulations ranging from serving warrants on potentially violent suspects, to getting pelted with paintballs while rushing to save an officer who's been injured.

"We have to stay on top of our game," says one undercover agent. "Lives are at stake."

They train together at least twice a month, while still working full-time on criminal investigations. One of the most crucial skills in HSI is knowing how to shoot.

"They have to shoot at least 90 percent or better on all the weapons we have," says Mike Kennedy, Assistant special agent in charge.

There's a part of this training that is very personal for these agents: practicing for months at a time, getting used to one another as a team, while driving an armored vehicle to a simulated scene where an officer has been shot.

It may just be training, with fake bullet holes on the victim and paintballs used instead of live ammunition, but it hits home all the same.

"We want to get it right," an undercover agent says.

Because of sequestration, the Department of Homeland Security may have to cut up to $4 billion in fiscal year 2013. In a speech last month, Department of Homeland Security Secretary Janet Napolitano said, "Sequestration would reduce HSI's activities, including human smuggling, counter-proliferation, and commercial trade fraud investigations."

Kennedy believes training would likely be among the first of HSI's activities to be impacted. He says agents would use far less ammunition in training, since the costs add up quickly.

One thing that will stay the same, regardless: the camaraderie.

"We have each other's backs, and I think the teamwork is probably the most fulfilling part," he says.