The messages have surfaced online from what appears to be a female minor in Los Angeles -- who says the 18-year-old suspect DM'd her on May 12 out of the blue and for no apparent reason at all ... explaining she has no idea who he is or why he reached out.

The alleged shooter tagged her in a photo he posted of his guns -- including what seem to be semi-automatic rifles that resemble AR-15s -- and then proceeded to start a private chat.

Of the many disturbing DMs he sent, this one was the most chilling ... "I got a lil secret I wanna tell u. I'm about to..." Apparently, he never revealed his exact intentions -- dancing around the subject/playing coy as the girl engaged him ... totally confused and flabbergasted.

The man went back and forth with her for a while, telling her she oughta be grateful he tagged her -- and then on Tuesday, he said this ... "Ima air out." The girl clearly didn't know what he was talking about, and seemed to acknowledge as much with follow-up questions.

In the aftermath of the onslaught Tuesday ... the girl revealed these messages and defended herself -- insisting she had no idea what this guy would do, and explaining she only engaged him to try to deter him from committing a horrible act.

The shooter, who we're not naming, shot his grandmother before heading to Robb Elementary School in Uvalde, TX ... where he barricaded himself in classrooms and opened fire, eventually getting taken out by law enforcement.

He's said to have been armed with a rifle and a handgun during the bloodshed ... and apparently loaded up on a ton of heavy artillery on his 18th birthday, all of which seem to have been obtained legally.