(NewsNation Now) — Police are speaking with four people of interest in connection with the investigation of six people who were found dead last weekend in a Milwaukee home, police said Thursday.

No charges have been referred to the district attorney’s office, and the four individuals’ involvement, if any, was not immediately clear, Milwaukee Police Chief Jeffrey Norman said. Each of the people in custody is an adult.

Police did not release the names of the people in custody or allude to how they might be tied to the situation, which investigators referred to Thursday as a “mass shooting.”

The investigation stems from a call for a welfare check at about 3:45 p.m. Sunday. Four men and one woman were found dead inside the home in the 2500 block of North 21st Street, according to police.

Police are investigating the deaths as multiple homicides, authorities said. The time the incident occurred is undetermined.

The victims had injuries that are suspected to have been caused by gunfire, Milwaukee Police Sgt. Efrain Cornejo said in an email to The Associated Press.

The Milwaukee County Medical Examiner’s Office tweeted early Monday that the body of an additional person, a man, was recovered from the home. The identities of the dead were pending.

On Monday, police became aware of a 911 call that was placed about 12 hours before investigators discovered the victims’ bodies.

Although the call was considered to be related to the investigation, it wasn’t directly tied to the address where the incident occurred, Norman said.

The 911 caller claimed she was a victim of a shooting and that multiple other people were shot, Norman said.

The caller gave police two different addresses in the vicinity of where the homicides occurred, but officers didn’t locate evidence of a shooting at either location, Norman said.

“Eventually, we were able to locate the person who made the call and we were able to determine she was not a victim of the shooting,” Norman said. “Her connection to this incident still remains under investigation.”

Police and local government have asked the community to come forward with any information related to the homicide.

“Every person in the City of Milwaukee is responsible and has a part in public safety. In this case, I’m certain that there’s somebody out there, somebody who knows something, who’s seen something, who can say something to help us in this issue where there was a mass shooting, and six people in our city were left dead,” Milwaukee Mayor Cavalier Johnson said Thursday. “It’s time for that person to step forward, to come up to say something.”

This is a developing story. Check back for updates.