Atlanta Airport Armed Suspect's Friend Said He Reached Out Day Before Arrest

A friend of Billy Joe Cagle -- the man arrested Monday at Atlanta's major airport for allegedly threatening a mass shooting -- is now talking to TMZ ... revealing the red flags that pushed him to reach out the night before cops swooped in.

Nick Roberts joined us on "TMZ Live" Tuesday ...  telling us he hit up Billy on Sunday night after spotting some troubling Facebook posts ... ones that, at first, looked more like warnings of self-harm than threats against others.

Nick said during that chat the 49-year-old seemed totally fine, giving off no sign he was plotting anything for the next day. Cops, though, were tipped off by Cagle's family’s concerns ... and thankfully, any potential tragedy was avoided.

You gotta catch the full clip ... 'cause Nick turns the ordeal into a positive reminder -- always check in on your friends, especially when their behavior feels off from the norm.

Nick highlights how Billy Joe has always been a caring, hardworking guy ... but he describes Cagle as "just a desperate man reaching for help, I felt like" ... underscoring the seriousness of mental health and how difficult in can be to process for yourself, and your loved ones.

Cagle was taken into custody Monday at Hartsfield-Jackson Atlanta International Airport -- where cops say they found a semiautomatic weapon and 27 rounds of ammo in his truck, WXIA-TV reports. Atlanta's mayor said Cagle was having "mental challenges" ... Atlanta's police chief said Cagle arrived at the airport just after 9 AM Monday after allegedly livestreaming he was going to "shoot it up."