By Michael Gold, William K. Rashbaum and Daniel E. Slotnik

The killer, dressed entirely in black and wearing a black mask, followed the young technology entrepreneur from the elevator of his luxury condo building into his apartment.

Then he used an electrical stun gun to immobilize the entrepreneur, Fahim Saleh, detectives believe.

Some time after, the assailant killed Mr. Saleh, decapitated him and dismembered his body with an electric saw.

The investigation was in its early stages, but that was the chilling account that a law enforcement official briefed on the inquiry gave on Wednesday as detectives continued scrutinizing evidence in the shocking killing of Mr. Saleh, 33. His body parts were found Tuesday in plastic garbage bags in his apartment on Manhattan’s Lower East Side.

On Wednesday, the police continued their investigation at the building and inside Mr. Saleh’s apartment, a seventh-story unit that he bought last year for $2.25 million, public records show, and for which he expressed his affection on Instagram.

The investigation included a review of surveillance video from Mr. Saleh’s apartment building and an interview with his sister.

Investigators believe the killer’s work dismembering the body was interrupted when the victim’s sister entered the apartment to check on him after not hearing from him for a day, another law enforcement official said.

Detectives think that the assailant fled through the apartment’s back door and into the building’s stairwell when she arrived, the official said.

A Police Department spokesman said he could not comment on how the killer entered the building. The property’s management company said that the building did not have a doorman but had extensive security in place.

The management company, in a statement, said Mr. Saleh was an active member of the condominium board.

“The unit owners, residents, board and management are all quite upset,” the statement said.

On Tuesday, a law enforcement official said that the electric saw was still plugged into an electrical outlet when the police arrived, and the killer had left some cleaning supplies behind. It appeared that some effort had been made to clean up the evidence.

The official who spoke on Wednesday said that the killing “looks like a professional job,” referring to it as a “hit.” A Police Department spokesman said he could not comment on a possible motive or whether any suspects had been identified.

On Tuesday night and Wednesday morning, detectives were scouring the neighborhood for surveillance video from outside local stores, residential and commercial buildings and area traffic cameras to see if cameras had captured the killer coming or going or waiting for his prey, several law enforcement officials said.

The grisly death has attracted international attention given Mr. Saleh’s global connections. The son of Bangladeshi immigrants, Mr. Saleh founded ride-hailing companies in Bangladesh and Nigeria as well as a venture-capital fund based in Manhattan that invested largely in companies in the developing world.

On Wednesday, Bangladesh’s minister for information and communication technology, Zunaid Ahmed Palak, expressed his condolences and said on Twitter that Mr. Saleh’s death was a great loss for the country.

“Fahim found success at an early age and built on it year after year, while remaining grounded and committed to helping others,” Mr. Saleh’s family said in a statement. “No matter what he did, he did it while thinking of the greater good and his family. His parents and his sisters were his light and he was theirs. There are no words or actions to provide any of us comfort except the capture of the person who exhibited nothing short of evil upon our loved one.”

Sumeet Rametra, who met Mr. Saleh in college and has been in close communication with his family since his death, described his friend as someone who indulged in kind gestures, like buying his parents a home and a Tesla.

On one recent occasion, Mr. Saleh asked Mr. Rametra to join him for a game of tennis, but Mr. Rametra said he didn’t have a racket, he recalled.

“He was like, ‘I already got you one, let’s go,’” Mr. Rametra said, crying. “I never got to play with him.”

Mr. Rametra, 31, also lauded Mr. Saleh’s business acumen, saying that his friend was a visionary who was always looking to his next idea.

“He was a machine, dude, he never stopped,” Mr. Rametra said. “He was always trying to make money.”

Mr. Saleh grew up near Poughkeepsie, N.Y., according to public records. As a teenager, he learned to code and began to develop websites, his friends said. He graduated from Bentley University, a small college in Waltham, Mass., in 2009 with a degree in computer information systems, according to his LinkedIn profile.

On social media, in interviews and blog posts, Mr. Saleh depicted himself as an entrepreneur driven by passion.

“Entrepreneurs are the ones that really change countries, that really change cities,” Mr. Saleh said in a YouTube video in February. “They’re the ones that bring the vision.”

After college, Mr. Saleh turned his love for practical jokes into a prank-calling app named PrankDial, which allowed users to buy prerecorded calls to send to friends.

Writing years later about the business, Mr. Saleh said that it eventually generated millions and led him to realize that he could keep turning his passions — in this case, for practical jokes — into big bucks.

“If you go into a project entirely focused on making money, you’re going to be disappointed,” he wrote in a post on Medium.

Still, PrankDial had its stumbles. On his LinkedIn profile, Mr. Saleh boasted that while his company had millions of downloads, it also drew more than 100 subpoenas.

At the time of Mr. Saleh’s death, PrankDial and its owners faced an active civil suit from a New Jersey jail worker who was convicted in 2015 of using the site to illegally wiretap his co-workers.

After PrankDial, Mr. Saleh turned his attention to Bangladesh, where he co-founded the ride-sharing company Pathao in 2015. The business, which Mr. Saleh left in 2018, started as a bike-sharing company but now offers transportation, delivery and business logistics.

“Fahim believed in the potential for technology to transform lives in Bangladesh and beyond,” Pathao said in a statement.

Fueled by his success in Bangladesh, Mr. Saleh tried to launch a similar venture in Nigeria. That company, Gokada, began operating as a motorcycle ride-hailing company in Lagos, Nigeria’s most populous city, in 2018.

Motorcycle taxis, called okada in Nigeria, have long been popular in Lagos and many other African cities as a way to circumvent traffic jams. Gokada raised $5.3 million in venture capital in June 2019, according to the website TechCrunch.

But Mr. Saleh’s business hit a major stumbling block in February, when state officials banned motorcycle taxis from operating in major commercial and residential parts of Lagos.

Gokada was forced to halt its ride-hailing business. In a video discussing the ban, Mr. Saleh, the company’s chief executive, appeared crestfallen.

“This has definitely been a blow,” Mr. Saleh said in the video, which he posted to YouTube and social media platforms on Feb. 2 and titled “Gokada is not okada.”

Mr. Saleh was forced to lay off workers, friends said. But he tried to rebound, saying he still believed that Nigeria had enormous economic potential and that he could provide jobs to young Nigerians.

In February, Gokada pivoted quickly to a food and package delivery service. After the change, Mr. Saleh sounded an optimistic note.

In April, as the pandemic upended New York and Nigeria alike, he noted on Twitter that his company was well positioned to adapt to the economic shift.

“Now it seems like we had a two-month head start in one of the few thriving business sectors,” he said.

The law enforcement official said that investigators were exploring on Wednesday whether Mr. Saleh’s killing might be related to his business, noting the indications that his company had been hurting.

Even as his start-up encountered setbacks, Mr. Saleh had remained both hopeful and persistent.

“Have a very good feeling about 2020,” he said on Twitter on June 2.

Juliana Kim contributed reporting. Sheelagh McNeill and Kitty Bennett contributed research. (The NY Times)

Correction: July 16, 2020

An earlier version of this story described incorrectly how Fahim Saleh was shot. It was with a stun gun, not a single shot.