MORE DETAILS: Second Victim Dies In Waterloo Mass Shooting

A second person has died following a weekend mass shooting in Waterloo.

Police have identified the two victims as a 22-year-old man and a 28-year-old woman, but they have not yet released their names. The deceased were among 12 people who were injured early Saturday morning when gunfire broke out at 501 W. 4th Street, a former bar that allegedly was being used by some urban motorcycle clubs unlawfully as an after-hours establishment.

According to police, the downtown building was filled with an estimated 100 people around 3:15 AM Saturday when the shots rang out. Police believe the incident began as a gun battle between two or more people.

Authorities said 12 people were injured in the shooting, one of whom died at the hospital later Saturday morning. According to police, two victims were still receiving treatment at University of Iowa Hospitals and Clinics in Iowa City Monday morning. The remaining victims, who suffered a variety of injuries, had been released from the hospital.

In a Monday press release, police indicated that crime-scene processing was complete and authorities had obtained and served several court orders related to the shooting investigation. Officials said one court order resulted in the arrest of 25-year-old Sir-Frank Nelson III of Waterloo, a felon who was wanted on unrelated warrants. Police arrested Nelson on a charge of illegal possession of a firearm.

The investigation into the shooting is ongoing, police said. They continue to seek out, and speak with witnesses about the shooting. Authorities asking anyone who was in or near 501 W. 4th Street around 3:15 AM Saturday to contact Waterloo Police at 319-291-4340, Option 3.

Investigators are asking anyone with videos or photos of the shooting to access an online portal that Waterloo police have set up and submit what they have. To submit digital evidence to police, CLICK HERE. According to authorities, the information can be submitted anonymously.

Cedar Valley Crime Stoppers is offering a $2,500 reward for information leading to an arrest in the case. Anyone with information should call Waterloo Police at 319-291-4340, Option 3, or Cedar Valley Crime Stoppers at 855-300-TIPS (8477). Tips may also be left at WWW.CVCRIMESTOP.COM.