Police fired at least 14 times, Kelly said.
Kelly said that some of those injured in the incident may have been hit by police bullets, adding that the injured are expected to survive.
Kelly said Johnson was a designer of women's accessories at Hazan Imports until he was laid off in a downsizing.
Johnson bought the .45-caliber gun used in the shooting in 1991 in Florida, two law enforcement souces told NBC News. He was not licensed to carry arms in New York, they said.
He was wearing a gray suit and carrying a briefecase when he gunned down Ercolino, officials said.
Ercolino, who lived in Warwick, N.Y., was the father of a young boy, neighbors there told NBC News.
Witnesses described a chaotic scene Friday morning on streets crowded with tourists and commuters alike.
"People were yelling 'Get down! Get down!", said Marc Engel, an accountant who was on a bus in the area when he heard the shots. "It took about 15 seconds, a lot of 'pop, pop, pop, pop, one shot after the other."
"I heard pop, pop, pop, pop, and I ran back into my offices," Gloria Walker, another witness, told NBC News. "I ran, I ran, I ran."
The brother of a woman who was shot in the leg told reporters she had been heading to Dunkin' Donuts when she heard the gunfire. While trying to decide whether she should duck or run, she was struck, her brother said.
"She's fine -- just a little shook up," he said. "Other than that, she's fine."
Those hurt ranged in age from 20 to 56. A tourist from North Carolina was among them.
The FDNY told NBC News they responded to a call about the shooting at Fifth Avenue and 34th Street at 9:07 a.m. EST Friday and arrived at 9:13 a.m EST. The body of the gunman remained on the street, under a white sheet, in front of Heartland Brewery until it was taken to the city medical examiner's office around noon.
The Empire State Building operators said the building was not involved in the shooting and remained open.