FIRST ON 3: TMZ.com posts video of Jenelle in girl-fight

OAK ISLAND, NC (TMZ.com) – “Teen Mom 2” star Jenelle Evans ferociously ATTACKED another woman in North Carolina this week … while her scumbag friends cheered her on … and the brutal assault was all caught on tape.

19-year-old Jenelle is clearly the instigator — spewing all sorts of nasty insults at a woman named Britany … before the reality star suddenly socks the other woman in the face with a closed fist.

The two women tussle to the ground … where Jenelle continues to pummel Britany … even AFTER it’s clear she can no longer defend herself.

With Britany bleeding from her face, one of Jenelle’s scumbag friends pulls the reality star off the other woman … while Janelle continues to drop verbal insults.

It’s unclear why Jenelle wanted to fight Britany … but sources connected to the incident claim it had something to do with Jenelle’s boyfriend.

The Brunswick County District Attorney’s Office tells TMZ.com the matter is “under investigation” … noting it is “working closely with the Brunswick County Sheriff’s Department.”

CLICK HERE TO SEE THE VIDEO: http://www.tmz.com/2011/03/25/teen-mom-2-jenelle-evans-catfight-video-footage-britany-truett-fist-brawl/