The N.Y.C. subway was struggling to rebound. Then the Brooklyn shooting happened.

The New York City subway was already struggling. A pandemic-fueled ridership plunge left the system facing a fiscal cliff. A rise in random attacks scared off passengers in droves. The shift to remote work that seems poised to outlive the coronavirus may mean ridership never fully recovers.

Mayor Eric Adams and Gov. Kathy Hochul have spent months cheerleading the return of the subway. They also sent hundreds of police officers to stations to tame crime and force out homeless people. The number of weekday passengers was still more than 40 percent below pre-pandemic levels in March.

Then came Tuesday, and a subway nightmare the city had never before endured. As a crowded morning-rush train lurched along in Brooklyn, a man with a gun and several extended magazines of ammunition began firing at helpless passengers who had nowhere to hide.

With the gunman still at large, Mr. Adams ordered the police to flood the system with even more officers, but the attack seemed to underscore the limits of policing as a preventive strategy. Officers had patrolled inside the station several times Tuesday morning but were not there when the gunman struck, the police said.

In a disturbing video posted in January, a man whom law enforcement officials identified as Frank R. James, who was named a suspect on Wednesday morning and is now in police custody, bragged that it would be easy to get away with causing mayhem in the subway and mocked Mr. Adams’s efforts at reducing subway crime.

“He may slow it down, but he ain’t stopping it,” Mr. James says. “That means you’d have to police in every station and that’s just not possible.”

The subway’s own infrastructure issues may have slowed efforts to find the gunman. At least one security camera that could have captured an image of the gunman recorded nothing, the mayor said. A law enforcement official said that none of the cameras were in full operation.

All of which complicates the mayor’s and governor’s efforts to convince people that the subways are safe to return to. And it has a lot of New Yorkers who have spent their lives riding the trains thinking very hard.

“I’m terrified of taking the subway,” said James Lee, 33, who lives in the Flushing neighborhood of Queens and works near the subway station where the shooting took place. “I never take public transportation anymore in New York, period.”

Ellen Silbermann, who at 71 has lived through New York in the 1970s and 80s when crime was much higher than it is now, pronounced Tuesday’s events “sort of apocalyptic.”

But John Butsikares, a 15-year-old who was on his way to Brooklyn Technical High School when the train doors opened onto a scene of panic and smoke and blood, said he refused to let the violence deter him.

“We have to keep living our lives,” he said. Then he added, “I’m just hoping the police will keep us safe.”

Ashley Southall, Troy Closson, Isabella Grullón Paz and Alexandra E. Petri contributed reporting.