A Long Island CPA was busted by the feds Thursday for participating in the Capitol siege — after he posted incriminating pictures on Facebook, officials said.

Justin McAuliffe, 39, was arrested at his home in Bellmore, Long Island, Thursday morning for joining the Jan. 6 insurrection at the US Capitol that left five dead.

In the days after the violent assault, an ex-pal allegedly contacted the FBI by phone to turn him in.

The former friend shared images McAuliffe had posted on his Facebook page, which he deleted after the attack, showing him posing outside the US Capitol.

The FBI tracked down his Telegram account and discovered a picture of him hanging out in an unnamed Congress member’s office during the breach, court papers allege.

In the snap, McAuliffe, wearing square-rimmed glasses, and an unidentified man, with his feet propped up on a desk, flash peace signs. The men are flanking a large Trump flag.

The ex-pal had soured on McAuliffe after he posted claims on Facebook that “COVID-19 was a hoax,” according to the papers.

Another tipster reached out to the FBI on Jan. 8 and provided two screen grabs from McAuliffe’s Facebook account, officials said.

One is a snap of his cellphone settings showing that he was connected to the US Senate’s WiFi network.

The second is of messages McAuliffe exchanged with another person on Facebook. “Yeah, I was in one of the offices,” he allegedly admitted. “Some people were smoking a joint in the room, lol. Cops came in and weee [sic] like. Okay guys really? They didn’t even ask us to leave right away. They let us sit down and hang out and relax.”

McAuliffe, who runs a CPA business in Mineola, is charged in a federal complaint out of Washington, DC, with one count each of entering a restricted building and disorderly conduct. He is expected to make his first appearance Thursday afternoon in Central Islip federal court.