Four people were killed and seven hurt in stabbings in northern Illinois on Wednesday, authorities said.

A 22-year-old man is in police custody and was being questioned Wednesday afternoon, according to Rockford Police Chief Carla Redd. She said one of the people who was wounded remained in critical condition.

"My heart goes out to the families right now that are suffering a loss," Redd told reporters. She said the Rockford Police received a medical call at 1.14 pm, followed by additional calls for police and paramedics.

"We don't believe there's any other suspects that are on the run or at large at this particular time," Redd said. "Right now, we don't have a clear motive as to what caused this individual to commit such a heinous crime."

Not all of the victims found at multiple addresses in the city had stab wounds and none were shot, according to Redd.

Rockford Police initially said five people had been injured. Cori Hilliard, a public information officer with the Winnebago County Sheriff's Office, told AP Wednesday evening that two more victims were among those hurt.

Three people died at the scenes. The fourth died at a hospital. Police later identified those victims as a 15-year-old girl, a 63-year-old woman, a 49-year-old man and a 22-year-old man. Their names were not released.

Redd said residents in the area were being asked to review their home surveillance camera footage for anything related to the attacks.

Rockford's population is about 150,000 and it's 144.84 kilometres northwest of Chicago. The violence Wednesday came days after a teenage employee of a Walmart in Rockford was stabbed and killed inside the store.

"Today, we are shocked by another horrific act of violence against innocent members of our community," Rockford Mayor Tom McNamara said. Now that the suspect is in custody, he continued, "Our primary concern is ensuring that our community members directly impacted by this violence are supported throughout their healing and recovery."

The mayor wrote on the city's Facebook page that "multiple jurisdictions" are "working on multiple crime scenes to develop an understanding of what transpired in an effort to prevent this from happening again".

The suspect was arrested by a Winnebago County sheriff's deputy after they were called to a reported home invasion, Sheriff Gary Caruana said.

"The young lady ran from him," Caruana said of one of the survivors. "She got some stab wounds in her hands and her face. She is in serious condition. One of the good Samaritans stopped to help her out. He did get some stab wounds. He is being checked out."

Resident Eric Patterson said he was struggling to make sense of the violence on his street.

"You can't rationalise this," Patterson told the Rockford Register Star. "It's almost like playing a video game, but it's reality. It makes no sense. It's like Grand Theft Auto. 'I'm going to run over the mailman here. I'm going to stab a couple people there. I am going to go in this house over here.'"

Another resident, Vanessa Hy, told WREX-TV in Rockford that the experience of witnessing the arrest was unreal, "like a movie".

"All of the sudden, we heard police run up on both sides of the house screaming 'Stop! Get down!'" Hy told the station. "Then they ran into the backyard and after a few minutes we saw them bringing the suspect down the driveway in handcuffs and he was very bloody."

Cassandra Hernandez, another neighbour, told the Rockford paper that she is friends with one of the victims.

"You never expect this here," Hernandez said. "We have such great neighbours