Police said the couple’s son discovered their bodies after he had trouble reaching them.
According to court documents, McAlpine's DNA was found on evidence at the crime scene.
At least two cars were taken at the time of the robbery, so police knew they were looking for more than one person.
"At the time, it was a solid two for sure," council member Mike Polensek told News 5 Monday. "And we were very much under the opinion that there were three because the cars didn't walk off the lot themselves."
Then a tip came in from a UPS driver in the area.
He said he saw three men in the lot prior to the shooting.
"I have prayed every night that they find the individuals who did this," Polensek said at a news conference Monday.
Court records state Diggs and Keener participated in a plot that resulted in the robbery and deaths of the Kuznik's and their dog.
Keener was arrested first. He also had outstanding warrants for several charges including grand theft and aggravated theft.
He was accused of stealing a 2016 Nissan.
Shortly after Keener's arrest, police arrested Diggs.
"We have a huge problem in this city — a huge problem!” Kuznik said in a March 22 interview that was set to air at a later date. “You can’t get the police here. There’s either not enough officers, or too much crime or something’s going on to the fact that we can’t get any help or support here.”