Prosecutors say 16-year-old Brooke Higgins, one of the teens charged as an adult in the alleged murder plot at Mountain Vista High School, searched on a website called Armslist.com for weapons.
Prosecutors described the site as basically the Craigslist for firearms.
Denver7 found you can find any kind of weapon you want on Armslist.com, from machine guns to assault rifles to even an operational tank with live canon.
But is it legal?
"It is legal, for these sites to be brokers of buying and selling," said Paul Brown with the Bureau of Alcohol, Tobacco and Firearms (ATF) Denver Division.
On its website, Armslist.com makes clear it "cannot and will not be a party in transactions," which means the site leaves it up to the buyers and sellers to follow state and federal laws.
"There's certainly challenges as far as people wanting to do things illegally on these sites," said Brown.
Prosecutors did not say Higgins was able to purchase any weapons on the site.
Higgins could have bought a weapon only if a private seller was willing to break the law.
"At 16 she should not have been able to legally own a weapon," said ATF spokeswoman Lisa Meiman.
In Colorado, you have to be 18 to buy a shotgun and 21 to buy a handgun.
Under state law, a private seller is also required to have a licensed dealer perform a background check on a buyer before making a sale.
"However, guns are accessible in this country and people can find ways to purchase a gun and own a gun illegally," said Meiman.
So while sites like Armslist.com are legal, the ATF said it also monitors them to make sure there is no illegal activity.
"It is certainly a priority for the ATF," said Brown.
Denver7 reached out to Armslist.com for comment, but has not yet heard back.
The ATF said when it comes to owning weapons like an operational tank, people can buy them legally but it requires a much more rigorous background check, paperwork and taxes under the National Firearms Act (NFA).
