Authorities predicted months ago that Justin Pena — the homeless man who allegedly shoved a straphanger onto the tracks in Midtown last week — would become a serial subway menace, The Post has learned.

Law enforcement had told a previous subway-attack victim — a retired postal worker whom Pena allegedly slugged in the face on an F train in Chelsea on Jan. 16 — that the unhinged shelter resident would soon be back out on the street, free to terrorize again.

“They told us he’s going to get a slap on the wrist and be back on the street and do the exact same thing,” recalled Hermann Leung, the son of victim Anthony Lion. “It’s very upsetting.”

Ten months later, the prediction came true, police now say.

Pena, 23, allegedly sucker-punched a 36-year-old stranger multiple times on a platform of the 42nd Street-Bryant Park, and then shoved the randomly targeted man onto the tracks.

That victim was able to pull himself back onto the platform before a train arrived; he suffered only minor injuries to his knee and hands, becoming the latest in a spate of attacks on subway passengers by violent, mentally ill men.

In the nearly year-long stretch between allegedly beating up Lion, 73, and shoving the younger man to the tracks, Pena has cycled in and out of Bellevue, his mother’s home and the streets.

One place he did not go to was court. Pena, who is now back in Bellevue, has missed multiple court dates and still has not been arraigned, according to a spokesperson for the Manhattan DA’s office.

Pena spent four years — from age 18 until shortly before the January attack — in jail on a gun-possession rap, said the mom, Angela Pena, 62.

Once released, never got the intensive, confining help he needed to stay on the medications he takes for bi-polar and attention deficit disorder, his adoptive mom, Angela Pena, 62, told The Post.

“Society did not do nothing for my son,” she cried, after learning from a reporter that he’d been jailed in connection to a second attack.

“Help my son, please!” she begged. “If you help him, he would not get in trouble.”

Pena had remained in Bellevue for just two weeks after the January attack, she said. Within weeks of returning home, he was off his medication and threatening to kill her, she said.

“I told them, ‘Keep him in the hospital. If you keep him in a hospital he will get medicated and he will not get violent,’” she said from her two-bedroom Bronx apartment, where she saves good memories of her son in a shoebox of photographs.

“But in the street, he will not be medicated. He will not take his medication,” she said.

“He didn’t take it with me. He didn’t take it when he was a baby. What makes them think he will take it in the streets?”

She added, “They should’ve done something [to help Pena] after he went to jail, because he’s violent.

“But nobody did nothing. Society did not do nothing for my son! My son went to jail. My son is sick in the head. My son is sick in the head,” she cried, despondent.

“Put him in a place where he could get medicated all the time, because if you tell him, ‘Medicate yourself,’ he does not medicate himself. He does whatever he wants.”

Experts agree with Pena’s mom.

To blame, they say, is a lack of intensive, in-patient facilities for the estimated 90,000 untreated, seriously mentally ill people who instead cycle in and out of New York’s jails, hospitals and homeless shelters.

The vast majority are harmful only to themselves, experts at the city-based Mental Illness Policy Organization — but a single-digit fraction of these street people are violent and, save for brief stints in hospital psychiatric wards, unhelped.

The city squandered $1 billion over the past five years on the ThriveNYC “wellness” program while programs for the seriously mentally ill go unfunded, they complain.

Meanwhile, cops are asked to pick up the pieces by responding to emergencies — or else are targeted by the “Defund the Police” movement.

“The subway is not a temporary housing facility for the homeless or mentally ill,” notes retired NYPD Sgt. Joseph Giacalone, an author and professor at John Jay College of Criminal Justice.

“You can spend all the time and money in the world on social workers to handle the mental health problem, but if there is no place to keep those that need the help, then it will fail miserably,” he told The Post Saturday.

“In addition, some of these suspects were arrested on other crimes, only to be released immediately under the new bail reform laws,” he added.

Subway push victim (left) Anthony Lion and his wife Dorothy Lion.
Subway push victim (left) Anthony Lion and his wife Dorothy Lion.Robert Miller

“Cops have seen this crisis coming for years,” agreed Pat Lynch, president of the Police Benevolent Association.

“Every time we handle a job involving a seriously mentally ill person — something we do successfully thousands of times each year — we leave knowing that any help we can provide is just a Band-Aid.”

The most psychotic street people — those who won’t take their medication on their own, “need to be confined for their own safety and the safety of others,” Lynch said.

Instead, “we’re being offered a false choice between ‘lock them all up’ [in jail] and ‘do nothing,’ because a real solution would take time, creativity, money and hard work,” he said.

The family of Anthony Lion still hasn’t recovered from his January assault — and after hearing of Wednesday’s alleged subway shove from The Post, they are afraid that more victims could follow.

“Oh wow, that’s terrible,” Lion’s wife, Dorothy, 66, said Saturday of last week’s attack.

Lion speaks mostly Cantonese, and is suffering from dementia, she said in speaking for him.

“There are a lot of mentally disordered people” in the city, she worried.

“Somebody has to follow up with [Pena’s] mental disorder and maybe send him to someplace like rehab or the hospital and get him treatment,” she said.

“I hope there’s no more of this type of victim — I hope it won’t happen again.”

Additional reporting by Larry Celona