Investigators are hoping the projectile that struck and killed a former University of Northern Iowa softball player and injured her boyfriend as she drove on Waterloo highway will help them solve the mysterious case.

Waterloo police said investigators have received what they believe to a bullet, which was taken from the wound cavity of Adam Kimball. Doctors at University of Iowa Hospitals and Clinics recovered the fragment when they treated Kimball for an injury to his face. The 32-year-old Kimball was released from the Iowa City hospital on Tuesday.

In a press release, authorities said initial indications are that the projectile is a bullet. The shell will undergo further examination, something that's consistent with a criminal investigation.

The bullet shattered the side window of an SUV driven by Micalla Rettinger, while she was driving on the U.S. Highway 218 bridge over the Cedar River around 2:30 AM Sunday. The projectile struck her in the neck, before hitting Kimball. The 25-year-old Rettinger died shortly after stopping her vehicle near the Greenhill Road exit. Another man in the vehicle escaped injury.

Waterloo police revealed Wednesday morning that a dog was also in the SUV. The animal wasn't hurt.

Authorities are trying to determine where the bullet came from and who fired the shot. The caliber could give investigators some idea how far the bullet traveled before hitting Rettinger's jeep. Police have not released the type or size of ammunition.

According to the press release, an autopsy on Rettinger was conducted Tuesday by the Iowa State Medical Examiner's Office. The results still weren't available Wednesday morning.

Authorities are still seeking the public’s help in locating anyone who may have been in the area of U.S. 218 between Cedar Falls and Waterloo around 2:30 AM on Sunday. This includes both vehicle and foot traffic on or off the roadway, along with anyone participating in outdoor activities in the area at that time, police said.

Officials are continuing to follow up all leads submitted by the community. Police said investigators continue to review a multitude of surveillance and traffic cameras from the area.

Late Wednesday, the reward for information leading to an arrest in the case grew to $24,000. Tips can be called in to the Waterloo Police Department Investigations Division at 319-291-4340 Option 7, or Cedar Valley Crime Stoppers at 855-300-TIPS (8477).

Rettinger graduated from the UNI in 2016 with a degree in biology. She played softball for the Panthers from 2013-2016, starting every game each of the four years. UNI held a moment of silence in her honor prior to Tuesday's game against Iowa.