Police in Will County and the city of Joliet said they did not know of a motive for the killings, but said the man knew the victims.

The FBI's fugitive taskforce is assisting local police in the search for the suspect, who has been named as 23-year-old Romeo Nance.

The victims were found on Sunday and Monday at three separate locations - several hours after the authorities warned on social media that the man should be considered "armed and dangerous".

One of the people killed was found on Sunday in a home in Will County. Seven others were found on Monday at two homes in Joliet.

Authorities said they also believe another shooting in Joliet that wounded a man on Sunday is connected to the string of violence.

Joliet Police Chief William Evans said: "I've been a policeman 29 years and this is probably the worst crime scene I've ever been associated with."

Will County Chief Deputy Dan Jungles he did not know how long the people in the houses had been dead.

Evans said the victims found Monday in the houses were members of the same family.