The bodies were found Thursday afternoon, and authorities on Friday were investigating how they were slain.
It appears police discovered a grisly scene: The bodies showed signs of trauma and were found in different places throughout the single-family home, interim police Superintendent John Escalante said.
One of the dead was a child, probably between 10 and 12 years old.
"This is going to be an investigation that's going to take some time," Escalante said at a news conference in front of the house. "We haven't ruled out anything at this point."
Despite the macabre discovery, Escalante said police don't believe neighbors should be worried. Authorities are adding additional patrols in the area as a precaution, however.
Chief of Detectives Eugene Roy said police are treating the case as a homicide.
"It's also important to note that there (were) no signs of forced entry to residence," he told reporter. "The victims were not bound, and there did not appear to be any ransacking of the house."
Detectives in the neighborhood and crime lab personnel are working to identify those killed, but Escalante acknowledged it is "very probable that this is all the same family inside."
"There is a very real possibility that this is something all internal that was contained within the residence," he said.
Autopsies showed that all six deaths were homicides, authorities said.
One of the deaths was caused by multiple gunshot wounds, and that body was described as a middle-aged female, the Cook County Medical Examiner's Office said.
The other deaths were caused by assaults using sharp and blunt force, officials said.
Specifically, three of the deaths were caused by multiple sharp force injuries due to assault, and those victims were described as an older male and two young males, the medical examiner's office said.
The other two deaths were caused by multiple sharp and blunt force injuries due to assault, and those victims were a middle-aged male and an older female, authorities said.
Many in the working-class community in southwest Chicago are shocked.
"Everybody is just pretty much upset and scared," Markita Williams, who lives next door to the house where the bodies were discovered, told CNN over the phone.
"It's just one of those things I can't explain. Good family, good kids," she said. "I just don't know."
On Wednesday, Williams said she noticed that a van belonging to her neighbor was on the street. The city was doing some work, and she worried that the car would get towed.
She went up to warn the family and saw the blinds were shut.
"Everything was closed up tight," said Williams, who has lived directly next door for three years. "They always had their curtains open -- you could see directly into the house from the front and on the side."
She knocked on the door. No response.
Police showed up the next day. One of the residents failed to show up for work two days in a row, so a co-worker called 911, according to Escalante.
Rosa de la Torre told CNN affiliate WGBO
that she was friends with one of the victims.
"[She was] a good person, very happy," she said in Spanish.
Police came to Williams on Thursday and asked her if she'd look after Maria, the neighbor's dog.
Maria looked shaken up and hungry. And she had blood on her.
So they took her in, cleaned her up and fed her.
"I don't really do dogs, but you know, we're really trying to care for her," she said.