The alleged gunman in the Brooklyn subway shooting was arrested in New Jersey more than two decades ago for threatening his old boss after getting canned, officials confirmed.

Frank James, 62 — who was detained after appearing in federal court Thursday on terrorism charges — had repeatedly called his former employer in Fairfield, Curtiss-Wright, and vowed some sort of violence over his termination, according to the Essex County Prosecutor’s Office.

Spokeswoman Kathy Carter did not provide what was said on those calls but said, “We believe it rose to the level of terroristic threats.”

James, who was living in Irvington, NJ, at the time, was charged in the 1995 incident with one count of making terroristic threats and was later found guilty of a lesser charge, harassment, Carter said.

A judge sentenced James to one year’s probation due to his clean record at the time, Carter said.

James was arrested Wednesday in Lower Manhattan after calling the cops on himself — ending a more than 24-hour manhunt for the alleged triggerman in the Sunset Park mass shooting that left 10 shot and more than a dozen others injured.

The NYPD picked up James at least 12 times in New York City from 1984 to 1998 on charges ranging from burglary and possession of burglar tools to criminal sexual acts, cops said.

He also had three other arrests in New Jersey, but the details of those cases were not immediately known.