They have been identified as 53-year-old Angela Stewart and 16-year-old Mason Stewart.
Family of the victims tell us it was Mason's 16th birthday.
In an update, authorities said the officer received a call of a robbery in progress at about 8:42 p.m. Thursday at an ATM in the 1600 block of Cartwright.
According to police, a man armed with a handgun approached a victim and robbed them of $200, before fleeing in a silver Honda Civic.
The victim followed the suspect while talking to 911.
Meanwhile, the officer responding to that ATM robbery was heading eastbound in the 16-1700 block of Cartwright Road, when the Stewarts pulled out of a Dollar Tree parking lot. That's when the officer T-boned their vehicle, police said.
Mason had received his license recently and was driving, officials added.
He and his mother died at the scene.
Witnesses described the crash, saying they heard a loud sound, then saw the police cruiser crush the other vehicle and catch fire.
Hours after the crash happened, a person was found in the back of the police cruiser, authorities told ABC13.
Video shows first responders frantically pulling him out of the cruiser to give him aid.
However, police haven't been able to explain why that person was in the back of the patrol car or why it took so long to discover the person was there.
"That really got to me. Three and a half hours later they just now find him? That means they didn't check if anyone was in the car," said witness Kenneth Mitchell, who told ABC13 he was at the Dollar Tree when the crash happened.
Police said Friday morning they are still investigating why a person was in the cruiser as it goes against policy for officers to respond to calls if they already have someone in custody.
The officer was taken to a hospital in the Texas Medical Center, but has since been released.
The person in the back of his patrol car was taken to Ben Taub Hospital. Police said he was not arrested and his condition wasn't immediately known.
Missouri City police said the officer has been with the department just under a year. He came to Missouri City from a prior agency, where he had about two years of experience.
Authorities are investigating whether he had on his cruiser's lights and sirens before the deadly crash.
An investigation is also underway into the ATM robbery. Anyone with information in that case is asked to call the Missouri City Police Department at 281-403-8700.
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