Johnson, who had played basketball at Simon Gratz High School, was pronounced dead an hour later.
Henderson called 9-1-1 and 11 days later identified the shooter as Bobby Hoyle, 26, from Cedarbrook. Hoyle, who had spent time in state prison on two firearms-possession convictions, was then arrested.
But, on Aug. 10, just before Hoyle's preliminary hearing on murder and weapon charges - with Henderson as the star witness - Henderson dropped a bomb.
He privately told Assistant District Attorney Ed Cameron and police detectives that he could no longer identify Hoyle as the shooter, according to the victim's mother, Rochelle Horton-Nichols.
"All their mouths dropped," Horton-Nichols said, recalling a conversation she had with Cameron the next day. Henderson allegedly told Cameron and the detectives that he "couldn't see him [the shooter]. It was too dark."
Without strong-enough evidence to hold Hoyle, Cameron had to withdraw the prosecution, Horton-Nichols said.
"I said [to Cameron], 'How does a police officer all of a sudden recant his story? Was he threatened? Was he paid?' " she said she told the prosecutor.
"If he's [Henderson's] saying [Hoyle] wasn't the shooter, he let four months go past," she said, frustrated. "Now, [authorities are] telling me they have to start from scratch. This is upsetting."
Police Homicide Capt. James Clark said that recently he had the case transferred to his Special Investigations Unit. Police still believe Hoyle was the man who fatally shot Johnson, Clark said.
"The investigation is ongoing to try to prove that," he said.
Meanwhile, Hoyle remains locked up in the city's Curran-Fromhold Correctional Facility on active state parole and county probation detainers, said prisons spokesman Bob Eskind.
'I don't have anything to hide'
Lt. Kevin Long, of Internal Affairs, confirmed that an investigation had been opened on Henderson and said that investigators were "looking into the consistency of" his statements.
He said the D.A.'s office requested the investigation. Tasha Jamerson, spokeswoman for the D.A.'s office, would only say the investigation into the shooting is ongoing.
Cameron, the prosecutor, didn't return calls by the Daily News.
Henderson, 44, who began his police career in 1990, has been assigned at the police impound lot near the airport since December 2000. Before that, he worked in the 22nd District in North Philly.
"They can investigate me all they want," he said. "I don't have anything to hide. I don't know none of them people. I don't know the deceased. I don't know the shooter."
When asked why he had previously identified Hoyle as the shooter in a police photo spread, then said he could no longer identify him, Henderson said he couldn't comment.
A source familiar with the case said Henderson may have told detectives he saw the shooter "to bolster his importance."
"He's a guy who just wanted to seem bigger and more important, then when he realized he was the only" witness in court, he backed off, the source said.
At the time of the shooting, Henderson was off-duty and unarmed in T&C's "Hair" It Is! Barber Shop & Beauty Salon, according to an affidavit of probable cause that resulted in Hoyle's arrest.
"The officer looked out the barbershop window and saw a black male shooting a gun four or five times down towards the ground and saw the muzzle flashes," the affidavit says.
Another off-duty police officer was also in the barbershop at the time of the shooting and left through the back door, police sources said. Henderson said he didn't know the other officer.
Horton-Nichols said she also heard that her son was in the barbershop about two hours before he was shot.
Calls by the Daily News to the shop resulted in two hang-ups. During a visit by a reporter, one barber said he wasn't working the night of the shooting; another had no comment. The shop owner did not return a call to his home phone.
Internal Affairs had twice previously investigated Henderson on claims of verbal abuse by residents when he worked in the 22nd District. One complaint was determined to be unfounded.
The other was sustained when investigators determined Henderson used profanity in an exchange with a man to whom he had served a protection-from-abuse order.
A deadly $10 argument
Police have said Johnson was shot after defending a younger buddy who'd been accused of taking $10 that the alleged gunman misplaced at Lu Grocery store, at Colgate Street and Cheltenham Avenue, a couple of blocks from the barbershop.
During a scuffle outside the store, Johnson allegedly punched the man who accused his buddy.
"An hour later, my son is dead," Horton-Nichols said.
The gunman shot Johnson on the sidewalk outside Reish's Pub, on Rising Sun Avenue near Rosalie Street, across from the barbershop.
Johnson, who graduated from Simon Gratz, planned to go to community college, his mother said. She said he had a daughter, who is now 5 months old, and treated another child, the baby's half-sister, as his own.
Now, Horton-Nichols, who lives in North Carolina, has only the memories of her son. And she faces the nightmare that her son's killer may not face justice.
"They had put all their eggs in one basket," she said of the authorities' reliance on Henderson as their star witness. "They thought it was an open-and-shut case. They say if no eyewitnesses come forward, Bobby walks."