A man suspected of abruptly pulling a gun and killing a stranger on a New York City subway train was arrested Tuesday, with police saying his motive for the unprovoked attack was “a big mystery.”

Andrew Abdullah, 25, was expected to face a murder charge in the death of 48-year-old Daniel Enriquez, who was shot to death while heading to Sunday brunch.

Abdullah "targeted this poor individual for reasons we don’t know,” Chief of Detectives James Essig said at a news conference.

The arrest came hours after police posted Abdullah's name and photo on social media and implored the public to help find him. But after the arrest, police disclosed that officers briefly stopped him after the shooting but let him go because his clothes didn't match the description they were given.

The Legal Aid Society, which is representing Abdullah, said it was just beginning to review evidence and urged the public not to make assumptions about the case.

“Mr. Abdullah deserves vigorous representation from his defense counsel, and that is what The Legal Aid Society will provide,” the organization said in a statement.

About six weeks after another subway shooting wounded 10 people, witnesses Sunday saw a man pacing the last car of a Q line train heading from Brooklyn to Manhattan, muttering to himself, Essig said. The only words witnesses could make out: “No phones.”

Then the man pulled out a gun and fired at Enriquez at close range, hitting him once in the chest, police said. The shooter fled after the train arrived at Manhattan’s Canal Street and ditched his gun by handing it to a stranger on the subway stairs, Essig said. Police eventually found the recipient and the gun, which had been reported stolen in Virginia in 2019.

About a block and a half away, officers stopped Abdullah and asked him what he was doing, Essig said. But he wasn't wearing the black hoodie mentioned in the initial suspect description, and he had a backpack that hadn't been mentioned. Officers let him leave but took down his name.

Only later, when viewing surveillance video, did police realize that the gunman had removed the sweatshirt after the shooting, Essig said.

The Legal Aid Society said it had tried since Monday night to arrange for Abdullah to surrender in the subway shooting, but authorities instead made a “completely unwarranted and inappropriate” decision to apprehend him outside the organization's office. An inquiry was sent to police.

Abdullah was on parole until last June after serving 2 1/2 years behind bars on a conviction on conspiracy and attempted weapon possession charges in a gang case, according to parole records and police. Court records show he has open criminal cases stemming from an April 24 vehicle theft and an alleged assault in 2020. He hasn't entered a plea in either of those cases; messages seeking comment were left with his lawyers.

“This horrific crime should never have happened,” Police Commissioner Keechant Sewell said at a news briefing Tuesday, calling Abdullah “a repeat offender who was given every leeway by the criminal justice system.”

Before his arrest, Enriquez’s sister Griselda Vile implored the city Tuesday to tackle crime more effectively.

“I’m pleading that this not happen to another New Yorker,” she told Fox News. “I don’t want my brother just to be a passing name in the media, a passing name in our normalcy post-pandemic.”

Enriquez worked for the global investment research division at Goldman Sachs, where CEO David Solomon called him a dedicated and beloved employee who “epitomized our culture of collaboration and excellence.”

A child of Mexican American parents, Enriquez spent his early childhood in Brooklyn before his family moved to California and then to Seattle, his partner, Adam Pollack, told the New YorkPost.

Enriquez returned to New York City in the mid-1990s to pursue a master's degree in Latin American studies at New York University. His yen for learning didn't stop there - during the coronavirus pandemic, he learned to play the guitar and to speak Portuguese and Italian, his family and partner said.

“He was constantly in self-improvement mode,” brother-in-law Glenn Vile told Fox News.

The eldest of five children, Enriquez texted his siblings about an hour before he was killed to advise them to check on their parents, who have health problems, she said.

The seemingly random shooting further shook a city already on edge about public safety. Many types of crime have rebounded after dipping dramatically earlier in the pandemic when people were staying home.

In the first five months of 2022, the number of shootings in the city dropped slightly over the same period a year earlier, and the number of murders is down 12% so far over last year. But New York is still on pace to have its second-highest number of homicides since 2011, after nearly a decade of record lows.

In terms of violent crime, the city remains substantially safer now than it was during the 1970s, '80s, '90s and early 2000s. But crime is now city voters’ top concern by far, according to a Quinnipiac University poll released this month. It surveyed 1,249 registered city voters and has a margin of error of +/- 2.8 percentage points.

Mayor Eric Adams, a Democrat who campaigned on promises to make the city safer, said his administration will evaluate how it is deploying officers across the sprawling subway system.