Police said Johnson hid behind a car and then killed Ercolino with five gunshots as he arrived for work. Johnson then was shot by two police officers who confronted him on a busy sidewalk.
Security camera footage showed the officers had only an instant to react when Johnson suddenly turned as they approached and pointed his gun at them, his arm cocked as if to fire.
Their encounter was over in eight seconds. The officers, who had been standing nearly close enough to shake hands with Johnson and had no opportunity to take cover, fired almost immediately.
Nine bystanders were wounded in the 16-shot volley, likely by stray or ricocheting police bullets. None of their injuries was life-threatening, police said.
Police investigating Johnson's killing of Ercolino were eyeing bad blood between them from when they worked together at Hazan Import, a garment district business where Ercolino was a vice president of sales.
Johnson and Ercolino had traded harassment accusations when they worked together, police said, and when Johnson was laid off from the company a year ago he blamed Ercolino, saying he hadn't aggressively marketed his new T-shirt line.
After Johnson's layoff, neighbors said, he continued to leave his apartment every day in a suit.
Internet records listed Johnson as the administrator of the website for a business called St. Jolly's Art, which sold iron-on graphic art for T-shirts. Art for sale on the site included stylized drawings of fighter planes and muscle cars and whimsical "seafaring vignettes" featuring pirate maidens and tall ships.
Johnson also was part of a community of bird watchers and photographers who document hawks and other wildlife living in Central Park, a few blocks from his home.
In one email to another bird watcher who works at The Associated Press, Johnson wrote tenderly about spending a winter night watching ducks in the park.
"Near midnight by the Harlem Meer I watched a little 'flotilla' of Mallards swimming and softly honking ... fifteen degree temp and they were carrying on unfazed. Just remarkable," he wrote.
His photographs of Central Park's hawk population appeared regularly on blogs tracking the birds.
A neighbor who often saw Johnson, 58, said he was always alone.
"I always felt bad," said Gisela Casella, who lived a few floors above Johnson in a modest apartment building on the Upper East Side. "I said, 'Doesn't he have a girlfriend?' I never saw him with anybody."
Ercolino, 41, was described by his relatives as the opposite of a quiet loner.
His eldest brother, Paul Ercolino, said he was a gregarious salesman who often traveled, had a loving girlfriend and was the life of any family gathering.
"He was in the prime of his life," he said. "He would do anything for anybody at any time. ... He was so wonderful with my children. At Christmastime, he was the one who always had the best presents for the kids."
Paul Ercolino said his brother, known to nieces and nephews as Uncle Ducky because of his nearly blond hair, had followed his father into the garment industry after growing up in Nanuet, just north of New York City, then later worked in women's handbags and accessories. He said his brother had never mentioned to the family that he had any problems with a co-worker.
Hazan Import Corp. executives didn't return phone calls seeking comment Friday.
Johnson, after waiting for Steve Ercolino to come to work, walked up to him, pulled out a .45-caliber pistol and fired at his head, police Commissioner Ray Kelly said. After Ercolino fell to the ground, Johnson stood over him and shot four more times, a witness told investigators.
"Jeffrey just came from behind two cars, pulled out his gun, put it up to Steve's head and shot him," said Carol Timan, whose daughter, Irene Timan, was walking to Hazan Imports at the time with Ercolino.
In security camera footage released by the police, Johnson can be seen walking calmly down the sidewalk after the shooting, distancing himself slightly from the other pedestrians, who appear to have no awareness that anything is wrong.
But when two police officers approach in a hurry, Johnson turns and pulls a handgun from a bag. Then, the scene explodes into action. People seated on a bench behind the gunman and pedestrians standing close to the two officers run for their lives.
Only a young child seems not to react, strolling out of view of the camera as adults all around leap away in terror.
Startled New Yorkers later looked up from their morning routines in the crowded business district to see people sprawled in the streets bleeding and a tarp covering a body in front of the tourist landmark.
"I was on the bus, and people were yelling 'Get down! Get down!" accountant Marc Engel said. "I was thinking, 'You people are crazy. No one is shooting in the middle of midtown Manhattan at 9 o'clock in the morning.'"
It was over in seconds, he said "a lot of pop, pop, pop, pop, one shot after the other."
Afterward, he saw sidewalks littered with the wounded, including one man "dripping enough blood to leave a stream."
The officers who fired were part a detail regularly assigned to patrol landmarks such as the 1,454-foot-tall skyscraper since the Sept. 11 terror attacks, officials said.
Kelly, the police commissioner, said the officers who confronted Johnson had "a gun right in their face" and "responded quickly, and they responded appropriately."
"These officers, having looked at the tape myself, had absolutely no choice," he said.
A witness had told police that Johnson fired at the officers, but authorities say ballistics evidence doesn't support that. Johnson's gun held seven rounds, they said. He fired five times at Ercolino, one round was still in the gun and one was ejected when officers secured it, authorities said.
A loaded magazine was found in Johnson's briefcase.
Johnson legally bought the gun in Sarasota, Fla., in 1991, but he didn't have a permit to possess it in New York City, authorities said.
Mayor Michael Bloomberg said New York still is the safest big city in the country, on pace to have a record low number of murders this year.
"But we are not immune to the national problem of gun violence," he said of the shooting, following mass shootings at a Colorado movie theater and a Sikh temple in Wisconsin.
The nine people wounded outside the Empire State Building were all from New York City, except for a woman from Chapel Hill, N.C. They suffered graze wounds or other minor injuries.
Metal detectors and bag searchers have been standard at the Empire State Building since 1997, when a gunman opened fire on the 86th-floor observation deck, killing one tourist and wounding six others before fatally shooting himself.
The skyscraper remained open Friday throughout the mayhem, although its workers became witnesses.
"We were just working here and we just heard bang, bang, bang!" said Mohammed Bachchu, a worker at a nearby souvenir shop.
He said he rushed from the building and saw seven people lying on the ground, covered in blood.
Rebecca Fox said she saw people running down the street and initially thought it was a celebrity sighting, but then she saw a woman shot in the foot and a man dead on the ground.
"I was scared and shocked and literally shaking," she said. "It was like 'CSI,' but it was real."
Contributing to this report from New York were Alex Katz, Samantha Gross, Julie Walker, David B. Caruso, Adam Geller, Karen Matthews, Ula Ilnytzky, Anne D'Innocenzio and Meghan Barr.