Police officers investigate a double homicide in the Kelvyn Park neighborhood on July 28, 2008 in Chicago, Illinois. The city's law enforcement has been using new technology called ShotStopper that can automatically detect shootings by using a network of sensors. Getty Images

Chicago's homicide rate was at the center of the national discussion about crime and violence after more than 125 homicides in the city in 2017, following a year in which the death toll reached 787. In January, President Donald Trump threatened to "send in the feds" to fix Chicago's "carnage." But instead of the feds, the Chicago Police Department has deployed new technology to reduce shootings in the beleaguered city, including sensors and microphones that can "hear" gunshots and immediately alert nearby police when guns are fired.

On Monday, Chicago continued its expansion of the system, called ShotSpotter, by opening new Strategic Decision Support Centers to support the technology and cover an additional 46 square miles of the city. Chicago has spent more than a million dollars on the system since rolling it out in 2012. New York begin using the technology in 2015 while paying $1.5 million a year. ShotSpotter is now being used by more than 100 police stations across the country, and its use raises a number of questions about surveillance, civil liberties and the future of policing.

Read: New Police Technology: Gun Cameras Device To Record Police Shootings

Here's how it works: Sensors equipped with microphone, GPS and transmitter, are placed 20 feet above the ground in high-crime areas. Fifteen to 20 of these sensors are placed in every square mile. This matrix of sensors can "hear" gunshots, and the company said the technology can distinguish gunshots from other loud noises, like fireworks and backfires from vehicles. The system is advanced enough that it can even identify the type of weapon used in a shooting.

The system then sends a notification to a command center, like the ones recently unveiled in Chicago, where the data is quickly verified, and then sent to police officers, where it can be delivered via smartphone, computer or dispatch.

The technology cuts down on law enforcement response times and provides police with precise information about the location and time of shootings, as well as the number of bullets fired and the weapons that fired them. But it also tells police a gun has been fired, because many times no human will. When ShotStopper was installed in Newark, New Jersey, police found that 30 percent all shootings in the city went unreported, Rolling Stone said.

ShotSpotter has insisted the microphones can't be used as listening devices, and only record when they detect gunshots. But civil liberty advocates can't help but be nervous about police putting microphones throughout the communities they are required to patrol.

"I am not losing sleep over this technology at this time," Jay Stanley, a senior policy analyst at the ACLU told DNAinfo last year. "But I am concerned over the precedent of allowing our cities to be sprinkled with live microphones that are not subject to transparent operation, and where that will lead over coming years and decades."

The technology's future will depend most on its effectiveness. While ShotSpotter CEO Ralph Clark said some cities have seen a 35 percent year-on-year drop in gunfire, some cities have decided the technology is not worth it's expensive price tag, the BBC said in March.

Dover, Delaware said the $195,000 the city was quoted for an annual contract would be better spent somewhere else. Charlotte, North Carolina used the technology, but decided not to renew its contract ShotSpotter after it didn't lead to the identification or prosecution of anyone who fired the shots detected by the system. And while Chicago continues to roll out the technology, the city's shootings show no signs of slowing down.