Police said they found the compound in the car of Stephen Paddock, the 64-year-old who gunned down 59 people and wounded over 520 others in Las Vegas Sunday night.
Ammonium nitrate was what made the explosion at the West, Texas, fertilizer plant in 2013 so destructive.
The chemical is typically used in fertilizers, cold packs, and explosives.
Takata used ammonium nitrate in its airbags, which is what turned them into "shrapnel", killing people around the world, The New York Times reported. The US Department of Transportation ordered the company phase out the compound by the end of 2018, or face even heftier fines than it's already had to pay.
Some countries are phasing out or banning the compound because of its potential use in improvised explosive devices, or IEDs.
But in the US, you can buy ammonium nitrate on Amazon, at gardening stores, or from agricultural suppliers.
The US Department of Homeland Security is required to "regulate the sale and transfer of ammonium nitrate by an ammonium nitrate facility" to prevent people from using it for terrorism.
While you may need to show an ID proving you're over 18, the chemical is surprisingly easy to get.
The site of the Route 91 music festival mass shooting is seen outside the Mandalay Bay Resort and Casino in Las Vegas, Nevada on October 2, 2017.
Along with the ammonium nitrate, 42 guns at Paddock's hotel suite and home, and several-thousand rounds of ammunition, investigators also found Tannerite, an explosive powder used for long-range target practice, at Paddock's home.
The company says on its website that the "KICK-A--" invention is legal under federal law to buy in all 50 states. It's a combination of ammonium nitrate and/or ammonium perchlorate (an oxidizer also used in rocket propellant) and a fuel so when a bullet hits the target, it explodes with a "VERY loud" bang.
More than 36 hours after Paddock carried out the deadliest mass shooting in modern US history, police still don't know his motive for killing so many people in such an orchestrated fashion.