NEW YORK — Subway workers began raising alarms early this year: Emergency brakes on New York City subway trains were being pulled deliberately, acts of apparent sabotage that were setting off major delays on the sprawling system.

There were clues. A man spotted surfing on the back of a train. The door to a rear cabin on another train was breached and the brake pulled. Another time, the safety chains on the back of a train were unhitched. The culprit also seemed to have a key to access train cabs.

Then, this week, after reports of cascading rush-hour delays on the 2 and 3 lines in Manhattan that had all the same earmarks, the Metropolitan Transportation Authority declared that a serial scofflaw was loose on the subway.

Officials say hundreds of trains appear to have been delayed since January, possibly by a single man who has pulled emergency brakes on dozens of trains, snarling the system. The result? Havoc for thousands of commuters.

The spree comes just as the subway — which has been hobbled by constant delays, breakdowns and aging equipment — has begun to show signs of getting better. But the sabotage highlights how, despite a major push to fix the system, the subway remains vulnerable to disruption, even from just one person pulling a brake.

The act is reminiscent of train stoppages in recent years by graffiti artists in cities such as Madrid and Auckland, New Zealand. In those incidents, graffiti taggers forced trains to a standstill using the emergency brake and then vandalized them. In February, Spanish police arrested 18 people for stopping and tagging four trains in Madrid’s underground tubes.

In New York, the transit agency and the Police Department have asked the public for help in stopping the saboteur. The brake puller has stopped about 40 trains, mostly in April and May in Manhattan, said Inspector Brian McGee, commander of the robbery division that is handling the case.

On Thursday, they released video of a young man surfing on the back of a northbound 2 train on Tuesday night, in the area of West 14th Street and Seventh Avenue in Manhattan, who they said had then pulled the emergency brake. After jumping off that train, the man then pulled the emergency brake on two other trains on the line, according to reports obtained by The New York Times.

Investigators believe the perpetrator has a key to access the controller stations located at the back of trains, which are normally locked and off-limits to passengers, allowing him to enter the controller’s chair, unseen by passengers. It is not yet clear whether the sabotage is the work of one individual, or a group.

“We have not seen something like this,” McGee said, adding that the police have increased patrols on train lines like the 2 and 3, and at stations across affected areas.

“It’s stupid. It’s dangerous. It’s selfish. And it’s got to stop,” Andy Byford, the subway leader, told reporters on Wednesday.

Subway officials said the incidents were more than just a prank and that the culprit was endangering subway riders, track workers and himself. Riders on stalled trains could have a medical emergency, for example, and subway tracks are notoriously dangerous because of the third rail, a steel column that carries 600 volts of electricity.

Transit officials said they had been reluctant to talk about the pattern of brake-pulling, fearing copycats. But after extensive delays during the afternoon rush on Tuesday, they decided to disclose what they knew.

A push by Byford to improve the system has shown results. The on-time rate for trains jumped to nearly 80% last month — up from about 68% a year earlier. Officials have focused on making repairs to the aging infrastructure and speeding up the trains.

While the brake puller is unlikely to have affected the on-time rate, he has made more than a few New Yorkers late, and now joins an infamous list of people who have deliberately interfered with train operations, either because of an obsessive interest in trains or simple malevolence.

Perhaps the best known is Darius McCollum, known as the “train bandit,” who has been arrested 32 times for impersonating transit employees, stealing trains and buses and driving their routes. He has a passion for trains and his exploits were turned into a documentary, “Off The Rails.”

The offender’s apparent use of a key to enter the train cab raised questions over whether he knew someone at the transit agency. How the offender obtained the key is one factor being investigated by transit officials.

Brake cords are also located inside train cars so passengers can use them in an emergency.

Transit officials said that they were analyzing dozens of incidents since the start of the year that involve factors like brake activations or “surfer” sightings to try and determine the actual scope of the brake-pulling spree. They are also reviewing security footage and sharing it with the police.

Each time, it was the same method, officials said. A man surfs on the back of the train, then likely uses a key to gain access to the rear operating cabin. He goes inside, pulls the emergency brake and then escapes onto the track and disappears into the darkness.

The result, Byford said, is a “double whammy.” The suspect not only stops the train whose brakes are pulled, but when he flees onto the tracks workers have to cut the power to look for him so even more trains are delayed.

Byford said the culprit — or culprits — were not “goofballs” but rather “morons.” He said he wanted harsher penalties for those committing these kinds of crimes. For now, the person or persons would likely be charged with reckless endangerment.

“I’d like to ban them from the subway,” Byford said.

According to the transit incident reports obtained by The Times, the emergency brake on three trains were pulled within an hour on Tuesday, causing delays to 80 trains. Also, the report said, a man made an “obscene gesture” to a train conductor who spotted him. On May 8, the emergency brake was pulled on a northbound 2 train near Prospect Park in Brooklyn. On May 14, it happened on a 2 train at West 28th Street. Jalopnik, a news website, first reported about the pattern of incidents.

On Wednesday, Kristin Myers, a reporter for Yahoo Finance, posted a video on Twitter of a man in a baseball hat riding on the back of a Q train. She said she had filmed it at 11:30 a.m. on April 27, at the DeKalb Avenue station in Brooklyn.

“I was in disbelief!” she said on Twitter. “Was right next to him and watched him climb onto the back of the train. He just stared at me. Like a commute-ruining psychopath.”

On the subway platform at Grand Central Station, riders expressed incredulity that anyone would make their commute even worse.

Given the chance, Joe Allen, 36, a delivery messenger who lives in Queens, said he would tell the brake bandit that he was “inconsiderate,” adding an expletive for emphasis.

He contemplated how he might have to tell his boss he was late because of the scofflaw.

“The Subway Brake Bandit struck again,” he said.

Riders also expressed concerns about safety. Lee Crousillat, who lives in Queens, said he worried that pulling the emergency brake could cause a derailment or that a falling passenger could injure his 3-year-old son, Lucien.

“Had he gotten hurt, I would be angry,” he said.

Andrea Jimenez, 31, who lives in Brooklyn, said she worried most about how someone was able to get a hold of a train key.

“I want to know how he got it,” she said.

This article originally appeared in The New York Times.