The exact cause of his death was unknown, and the question of whether the twin zappings played a role isn't likely to be clear until an autopsy is performed.
Police spokesman Lt. Frank Vanore said the encounter started to unfold shortly before noon, when police received two calls - one from Johnson - about a person with a weapon at 6925 Souder St.
The cops were plenty familiar with the address. "We had been there numerous times in the past," Vanore said.
"He had mental-health issues, and had been violent toward police before," he said, adding that Johnson had not been previously arrested.
Two officers who had received crisis-intervention training handled the call this time out.
When they arrived, Vanore said, the cops found Johnson darting in and out of his house, breaking things and grabbing sticks or tree branches, which he tried to set on fire by using the kitchen stove.
The cops ordered Johnson to stop. Instead, he lunged at them, Vanore said, and they fired a stun gun at him. "It had no effect," Vanore said.
Johnson continued to carry on, Vanore said, so police zapped him again, and he dropped to the ground. He died at Nazareth Hospital at 1:10 p.m.
When approached by a Daily News reporter last night, Johnson's family declined to comment. Earlier in the day, Michelle Rynkiewicz, his cousin, told CBS3 that Johnson was "severely retarded" and that a stun gun should not have been used on him.
Neighbor Beverly Douglas said Souder Street residents were used to seeing cops marching into Johnson's house.
"He's not too right in the head," she said. "The cops always come and take him out. He goes to a hospital for a few days and comes home."
Douglas said Johnson often walked around the neighborhood carrying small sticks, and was playful with small children.
"He was nice. He used to ask me for a soda," she said, before adding: "I couldn't believe that he died. It wasn't necessary to [zap] him twice."
According to Cyril Wecht, a prominent forensic pathologist from Pittsburgh, multiple stun-gun shots can have a cumulative effect. Just because a suspect doesn't fall from one shot doesn't mean his heart hasn't been affected, he said.
"When you bring in electricity - that's what you have with the Taser - you're directly compromising two vital systems of the body - the nervous system and the heart," Wecht said. "The first Taser, while not immobilizing, may well make the heart more susceptible to the effects of the second Taser" and make significant trauma more likely.
Wecht, who stressed that he didn't know all of the details of yesterday's incident, said that police must be carefully trained to use a stun gun.
"It's not something you use lightly," he said. "[But] generally, and to an overwhelming degree, it's relatively safe."
Without a Taser, some police might feel they had to use a gun, Wecht said.
Using a taser is "better than shooting," said Upper Darby Police Superintendent Michael Chitwood, who famously tested a Taser once in front of TV cameras. "[But] unfortunately people die. The electrical impulse is significant, but short term."
The important thing is using the taser correctly, Chitwood said.
"A lot of times you use these things and sometimes you can't take the people down because it's the wrong part [where the taser hit the body] or clothing [interfered]," he said.
The Philadelphia Police Department began offering crisis-intervention training to officers in 2007, after several mentally unstable people were fatally shot during confrontations with police.
The training focused on techniques to relieve tense situations - including the use of stun guns - and was developed with help from the Philadelphia Department of Behavioral Health and Mental Retardation Services and other mental health agencies.