Police said the two juveniles were located on Monday afternoon after receiving an "overwhelming" amount of information from the public.
A 15-year-old boy had earlier been charged and was set to appear in an Ipswich court on Monday following the fatal stabbing of the grandmother, identified as Vyleen White.
Ms White had been shopping with her six-year-old granddaughter at Redbank Plains west of Brisbane when she was stabbed in the chest in the centre's car park on Saturday.
She died at the scene a short time later.
The girl was unharmed.
The 15-year-old boy went to Ipswich Police Station on Sunday night and was charged with unlawful use of a motor vehicle.
"That does not mean there will not be further charges," Detective Acting Superintendent Heath McQueen told reporters on Monday.
Detectives have released CCTV footage of four youths in a Hyundai Getz, allegedly stolen from Ms White.
The search for the fourth youth continues but Det McQueen said police knew their identity.
Detectives from Ipswich and the Homicide Investigation Unit investigating the homicide of a 70-year-old woman at Redbank Plains overnight have released vision of four persons of interest, as investigations continue. — Queensland Police (@QldPolice) https://t.co/Op2IkIWy43 pic.twitter.com/5Slrdjh87iFebruary 4, 2024
He said police also wanted to speak with other "persons of interest".
"I am confident that we will apprehend those responsible for this offence and we will ultimately charge somebody with the murder of a 70-year-old grandmother," Det McQueen said.
"It doesn't matter where these people go. We will find them."
Asked about the motive behind the stabbing, Det McQueen said: "Sadly the motive was to steal a Hyundai Getz.
"I stand before you today talking to you about a murder of a grandmother in front of a six-year-old ... and the result was a stolen Hyundai Getz - that is the motive."
Det McQueen described it as a "callous, cowardly, violent crime".
"She was a loving, family woman," he said.
"She was taking her granddaughter to the shops. This is senseless violence."
Floral tributes for the 70-year-old have been left at the Redbank Plains shopping centre as family and friends mourn the loss.
A GoFundMe page organised by family friends had raised almost $11,000 by Monday afternoon.
"They have lost a mother, grandmother, a wife and loved family member of their family," the fundraiser page said.
"They are all so overwhelmed and so warm hearted by the community at such a difficult time."
Fundraiser Katherine Lameree said the page hoped to raise money for Ms White's family but also let the community know about the grandmother's legacy.
"She was such a selfless woman - with so much care for her family and absolutely any one she came into contact with," the page said.
"This is what they want her to be remembered by and not defined by the act of someone else."
Police remain at the scene of Saturday's attack. (Jono Searle/AAP PHOTOS)
Ipswich Mayor Theresa Harding on Monday said the attack had shocked the community, describing it as a "punch in the guts" to locals.
Redbank Plains residents are calling for more police presence at the shopping centre following the attack.
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