One of the children, a 15-year-old, called the Harris County Sheriff's Office on Sunday afternoon and told authorities that his 9-year-old brother had been dead for a year and the body was inside the apartment, the office said in a statement.
He also told deputies that his mom hasn't been back in months, leaving him and his two other brothers, ages 7 and 10, alone.
Our sister station KTRK-TV reports that before calling for help, the 15-year-old texted his mom to tell her he could not take it anymore.
SEE MORE: No charges filed, mother and boyfriend released after 9-year-old found dead and kids were abandoned
Neighbors in the apartment complex, like Erica Chapman, said the 15-year-old relied on them for food.
"The first time I saw him, he was sleeping on one of the slides," said Chapman.
Chapman said she started giving him food around six months ago.
She said she saw the boy's mother outside the apartment on rare occasions.
"I noticed his mom, a few times, would come and park, and he would run down and grab like noodles, drinks and chips and run back up," Chapman said.
But those items weren't enough so that's why Chapman would offer him food. She said, however, the teen wouldn't accept cooked meals.
"He would not eat anything that you made," Chapman said. "You would have to buy it - like it had to be a salad that was closed from the store or a pizza. Fruits still had to be in a bag."
Another neighbor, Trevor Thompson, said he would also provide the 15-year-old with food, starting about a week ago.
"A few times I noticed the lights weren't on, and it was quiet upstairs, so I figured there was trouble because the parents haven't been there in a while," Thompson said.
He said he started to give the 15-year-old food after the teen came to his apartment looking for help with his cellphone.
"One day he came down and knocked down on my door and asked if I could charge his phone. So I charged the phone. From there I built a bond with him," Thompson said.
Thompson described the teen as paranoid and said he was afraid of getting poisoned, which is why he would only accept certain food, as Chapman had noted.
"He wouldn't take cooked foods. He would take fruits and snacks," Thompson said.
Neither Thompson and Chapman were aware of the secrets happening inside the apartment. They didn't even know the teenager had two other brothers living with him.
Thompson said he started to wonder if there was more to the story after giving him a pizza.
"I asked, 'Did you like the pizza?' And he asked, 'The first pizza we had?'" Thompson said. "That made me wonder, why would he say, 'we?'"
Chapman thinks the 15-year-old kept his other two brothers a secret out of fear.
"After finding out what happened [Sunday], I think he was more nervous and scared," Chapman said. "(He might be thinking) 'are they going to blame me for this? Are my parents going to punish me for this?' Maybe his parents told him they would be separated if he told anyone."
Both neighbors said they never called authorities because they didn't understand the magnitude of the situation. They said the 15-year-old kept to himself, was quiet and didn't answer many questions.
"I didn't want to push him away by asking questions because I knew he was starving and needed food," Chapman said.
A TWISTED NIGHTMARE: 3 young boys were left abandoned and in squalid conditions, with their dead brother slowly decomposing for a year. I spoke with two neighbors who have been feeding the 15 y/o for 6 months. They never knew the secrets he had to live with. More @abc13houston pic.twitter.com/LhCsOkmhap— Brooke Taylor (@ABC13Brooke) October 25, 2021
According to the sheriff's office, all three boys were taken to the hospital, where doctors said they were malnourished with injuries. The 7-year-old had facial fractures, sources told KTRK.
Harris County Sheriff investigators said they are waiting for autopsy results to come in before filing charges.
Meanwhile, KTRK learned the mother and her boyfriend lived in an apartment just 15 minutes away from where her children were living.
On Monday, after the couple was questioned and released by the Harris County Sheriff's Office, KTRK tried to question them. They pushed up their windows and quickly drove off.
WATCH: Mom who abandoned 4 sons in Harris Co. apartment released by authorities
Neighbor Tanieka said the mother and her boyfriend lived in a unit downstairs and were frequently seen by other residents.
"We see them come and go," said Tanieka, who only wanted her first name used. "We never saw any children. None."
Tanieka estimated they have been living in this apartment since January or February.
Alief Independent School District confirmed to KTRK-TV that in 2019 and 2020, the school district filed truancy papers against the mother.
Two of her children apparently failed to show up to school. District officials said the kids were last enrolled in May 2020.
Officials were not immediately certain how many of the children were students in the district.
The pandemic has made matters worse for vulnerable children, according to Bob Sanborn, president and CEO of Children at Risk.
"It was very easy for these kids to fall through the cracks," Sanborn said. "Texas has always relied on teachers to be that line of first defense for child abuse and these kids never got to that place where a teacher could identify what was going on with them, or begin an investigation, or alert CPS (Child Protective Services) or others to what was happening."
The Texas Department of Family and Protective Services is seeking emergency custody of the three boys. A spokeswoman confirmed CPS has a history with the family, but there was no active investigation at the time they were found.