PLAGUED by a failing career, crushing debt and a frustrated wife begging him to ditch his showbiz dreams to find a regular job, former “Shield” star Michael Jace turned into a ticking time bomb, say sources.

Finally, the 6-foot-4 actor exploded in a fit of rage on May 19 and cops say he gunned down 40-year-old wife April mere feet from their two young sons.

As word of the alleged cold-blooded killing swept across the nation, The ENQUIRER launched an in-depth investigation to learn why Jace may have snapped – and we discovered that the 51-year-old actor had a terrifying violent streak he hid from his fans and neighbors.

“Michael and April were desper­ately in debt and escape seemed hopeless,” revealed a close source. “They were going to lose their house, and April told him she was thinking of leaving him. He was losing everything in the world that he loved.”

Jace’s career sputtered after “The Shield” was canceled in 2008, ending his long-running portrayal of inner-city police detective Julien Lowe opposite series star Michael Chiklis. Despite winning some guest parts on TV shows and a recurring role on another cop drama, “Southland,” he wound up filing for bankruptcy.

“WHEN HIS ACTING CAREER dried up, Michael hoped to become a screenwriter, but one of his screenplays was recently rejected, and I know that depressed him,” an insider told The ENQUIRER.

And his jujitsu instructor, Mark Robias, told The ENQUIRER: “He had high hopes for that story. He was extremely disappointed.”

Meanwhile, hard-working April, his wife of 10 years, was apparently fed up with trying to support their family on her small salary as a university financial aid officer.

“She nagged him to give up show business and get a real job,” revealed the close source.

But Jace could be stubborn – and violent!

During his 2002 divorce from his first wife Jennifer Bitterman, friends testified he slammed her into a wall and choked and beat her while a son, just 6 months old, watched.

But this was a side of Jace that he hid from neighbors, who remember him as a gentle giant. And April had gotten him involved in Christianity. She thought he was becoming a changed man… but cops say she was wrong.

The couple’s last bitter argument, which police say was over finances, occurred around 8:30 p.m. on the day of her death – with their sons nearby. Frightened neighbors ran for cover as blasts of gunfire split the air, and police arrived to find April’s bullet-riddled body in the hallway of their three-bedroom Los Angeles home. Now, Jace has been charged with murder and is slated to be arraigned on June 18.

“Michael lived the high life of a Hollywood success story, but that was all taken away,” said another close source. “He was washed up and, finally, I think the pressure got to him and he just snapped.”