Johnson said Thursday that, despite a department policy calling for the release of the name 48 hours after a shooting, he decided not to do so because of "serious safety concerns."
"Gaines' ideology, consistent with anti-government sentiment, is also a concern," the statement said. "While Gaines does not appear to have been actively affiliated with any specific anti-government group, she identified and behaved as a 'free person' who does not recognize governmental authority."
Gaines' Instagram and Facebook pages reflected the outrage in the black community over the latest episode in a wave of killings of African-Americans by police officers, according to media reports.
The 23-year-old Maryland resident was killed Monday after a seven-hour standoff with police. She was trying to live-stream the fatal encounter, authorities said.
'She believed in fairness'
Gaines' uncle, Jermaine Barnett, told CNN on Thursday that his niece's online presence reflected her sense of justice, not anti-government sentiments.
"She believed in fairness and felt that people weren't being fair," he said. "I never heard her say anything like 'f' the police or anything like that."
During the standoff, police requested that Facebook deactivate her account on the service and on Instagram, which Facebook owns, Johnson told reporters.
The Facebook account, which allows live-streaming, was taken offline, but two one-minute videos were still on her Instagram account on Tuesday afternoon.
The videos were posted just before Gaines was killed in an exchange of gunfire in which her 5-year-old boy was wounded in the arm.
In one video, a tactical officer appears in the doorway of an apartment. Talking can be heard but is hard to discern. There is no apparent conversation between the officer and Gaines. The video has since been removed.
In the other video, Gaines talks to the boy, who gives halting answers, which raises the possibility the boy might have been coached.
Gaines is heard asking the boy what the police are trying to do. He answers, "They're trying to kill us."
The boy was being treated for non-life threatening injuries at Johns Hopkins Hospital in Baltimore.
In Thursday's statement, police said investigators were trying to determine whether the boy was struck by a round or shrapnel from the officer's weapon or Gaines' Mossberg shotgun.
"They have not yet determined where the child was at the time of the shooting," the statement said.
'Anger and impulsive behavior'
The police chief said the homicide unit was investigating the shooting and a separate administrative review -- conducted for all shootings that involve police -- was underway. The findings will be turned over to the state's attorney for review.
Johnson said the "current national climate" was another reason for withholding the officer's name, referring to the recent shootings of officers in Dallas and Baton Rouge.
"Lone wolf attacks by people emotionally caught up in current events are a real possibility," he said. "We constantly balance the need for transparency with the need to protect investigations and safety. This is a situation where I feel we must err on the side of safety."
The chief's statement did not mention specific threats or examples of what he said was Gaines' anti-government ideology.
Meanwhile, new details about her life have begun to trickle out.
"She was really a social butterfly," said Barnett, who maintained contact with his niece via phone and social media. "I have never really seen her in any type of situation that was negative until this came along."
Another uncle, Jerome Barnett, told CNN: "She's a person that stood by what she believed in. She was a fair, loving person... She was about uplifting her people -- young black girls, African-Americans, minorities."
In 2012, Gaines filed a lawsuit in which she claimed that exposure to lead paint made her sick.
Gaines' blood tests revealed high lead levels from June 1993 to February 1994, according to court records.
"They played around the house," Gaines' mother, Rhanda Dormeus, said in deposition in the case. "Kids touch stuff, you know. At that age, they're hand-to-mouth all the time. So I would intervene if I saw them putting anything in their mouth."
A doctor who examined Gaines said in a court document that she reported "problems concentrating in school and this is still a problem for her."
He added, "Korryn had a history of problems with anger and impulsive behavior and had several sessions with her school counselor."
The suit, filed against the owner of two Baltimore rental homes she lived in as a child, was still pending.
"There is no report of a psychiatric or psychological assessment nor any specific diagnosis of mental illness," the doctor wrote.
On Monday, Gaines was posting video of the encounter with police as some of her followers urged her not to follow police orders, Johnson said.
Gaines had shotgun, police say
About 9:20 Monday morning, three warrant service officers went to serve a man and a woman in the Randallstown area of Baltimore County.
Upon arriving, the officers heard the voices of a man, a woman and a crying child inside the apartment.
They waited outside the door for about 10 minutes, Johnson said. Then, one officer obtained a key to the apartment.
The man, Kareem Courtney, 39, was wanted on an assault warrant while Gaines was wanted on a bench warrant for failing to appear in court to face traffic charges, police said.
Gaines had a 12-gauge shotgun, which she pointed at police, Johnson said. Records show she bought the gun last year.
The officers called for a tactical team and waited.
Not long into the standoff, Courtney left the apartment with a 1-year-old child and was arrested.
That left Gaines and her 5-year-old son in the home.
"Gaines was posting video of the operation as it unfolded," Johnson said.
Gaines vacillated between agitated and calm during the encounter, he said. Several times she pointed the shotgun at officers.
At around 4 p.m., hours into the standoff, the woman threatened officers verbally and with the weapon, according to the police account.
"If you don't leave, I'm going to kill you. I'm going to kill you," Johnson quoted Gaines as saying.
One police officer fired a shot and Gaines fired back with double-aught buckshot. Officers responded with three shots and the woman was struck.
Police didn't say why the first officer fired.
The 5-year-old boy was hit in the arm during the crossfire.
Social media deactivated
Gaines' Facebook account was deactivated, but nothing was deleted, said Baltimore County department spokeswoman Elise Armacost, emphasizing that police wouldn't want video deleted because it could serve as evidence.
Authorities are working to obtain warrants for records of social media accounts belonging to Gaines, who "clearly had anti-government views," Armacost said.
The officers will be placed on administrative leave, which is standard procedure in shootings involving officers, the police said.
Wanted on misdemeanors
A search of Maryland court filings shows that police filed traffic and criminal charges separately against Gaines in connection with a March 11 traffic stop. An officer issued Gaines citations related to her vehicle registration, insurance and tags -- offenses that would have commanded at least $560 in fines -- and filed criminal charges the day after the traffic stop, according to court records.
She was originally pulled over for driving without a license plate. In place of a tag, she had a cardboard sign with a handwritten message warning government officials not to compromise her right to travel, he said.
Gaines was accused of disorderly conduct, resisting arrest, littering and failing to obey a reasonable and lawful order, all misdemeanors. A failure-to-appear warrant was filed when she did not show up to court July 13, court records show.
Police: Gaines filed assault charge
Courtney, the man in the apartment with Gaines, was wanted on an assault warrant filed by Gaines following a domestic violence incident in "the weeks prior" to Monday's standoff, Johnson said. Details were not immediately available, but Maryland court records show that Courtney was arrested for second-degree assault on June 28 and his case remains active.
The chief said it was unclear why he only took one child when he came out of the apartment Monday. He was later released from custody.
Anger against police violence remains high in the Baltimore area, which is still reeling from the death of Freddie Gray, the 25-year-old Baltimore resident who died on April 19
, 2015, after suffering a spinal injury while in police custody.
CNN affiliate WBAL-TV reports
the case is the county's third officer-involved shooting of 2016 and the first fatal officer-involved shooting.