The gunman behind the Florida school massacre appeared to be feuding with people on Instagram — where he posted pics of his “arsenal” and claimed shooting was “group therapy,” according to reports.

An account purportedly belonging to suspect Nikolas Cruz, 19, features a number of disturbing images, including pictures of him holding up guns and knives — and captions making fun of Muslims.

The page was taken down after the deadly shooting, but screenshots were sent to The Post on Wednesday night by people who know the young man.

In almost every picture, he can be seen wearing a black shirt or scarf over his face, to conceal his identity.

“F–k youuuuuuuuuuuuu allllll,” Cruz wrote in one caption, from December 2015.

In other posts, viewed by the Orlando Sentinel and Miami Herald, the high school student appears to talk about background checks and buying a rifle — which he planned to outfit with a scope “for hunting,” the papers report.

One photo in particular shows a used paper target from a shooting range and the words “Group Therapy” underneath.

“It really does f–king work,” the caption said.

A post from 2015 — featuring a photo definition for the Arabic phrase Allahu Akbar, which means “God is great” — appeared to mock Muslims. It contained at least one slur.

Another post, from January 2016, shows an advertisement for a pump-action shotgun.

“I plan on getting this but I need more information on it,” the caption said. “If someone could give advice on how much I’m spending and background checks please to god let me know.”

The FBI has set up a website for social media users to report any other photos or videos that may be related to Wednesday’s massacre.

Cruz’s friends and former classmates have confirmed to multiple media outlets that the aforementioned IG page — @cruz_nikolas — is, in fact, his.

“Everything he posts is about weapons,” said one student, who spoke to ABC News.

“It’s sick.”