The toddler was later pronounced dead at a hospital, and now police say they are working to find out what happened.
Residents who saw the toddler being rushed to the hospital say they are still struggling with the memory of it all.
The two-year-old boy was found at the Empire Village apartment complex on Burke Road near Jenkins.
Police were called to the scene Friday afternoon after the child was found on the sidewalk. Eyewitnesses say the boy was bruised and wearing only wearing a diaper.
Pasadena police spokesman Vance Mitchell says officers arrived within two minutes, and found others at the apartment complex attempting to resuscitate the child.
Officers also administered CPR and transferred the boy to Bayshore Medical Center, but doctors were not able to save him. An autopsy will be performed to determine his cause of death.
Neighbors recall seeing the little boy upstairs, saying he was a happy child and would always wave at them as they passed by.
"It's sad," neighbor Santana Miranda said. "What can a little boy do to you for you to do something like that to him? I have kids of my own and it's just sad."
"I was crying. My girls were crying," neighbor Marybel Abrgeo said. "We would always see him upstairs."
Mitchell says investigators found bruises on the toddler's body, but have not yet determined how they got there.
Child Protective Services says it has taken two other children from the same apartment. The custody case is expected to go to court Monday morning.
Please stay with Eyewitness News and abc13.com for updates as we continue following this story.