For a month, Lopez believed Sibson intentionally killed her daughter in an attempted murder-suicide. Investigators later told her that only one bullet shell casing was found at the scene.
Sibson was not initially expected to survive the gunshot wound to his head.
"There was substantial doubt at the outset (that) Mr. Sibson would survive. He had an entry wound on the left side of his head, he had an exit wound on the top of his skull," assistant district attorney James Fayette said, according to KTVA.
Lopez and Haag’s sister, Chelsea Hartman, said the couple was happy and that the two were high school sweethearts who had been together for six years.
“I think a lot of it is still hard to grasp of how is it just one bullet, you know? And how does it go through one person and then the other person and kill the second person? It’s just not fair,” Hartman said.
Sibson pleaded not guilty to a charge of second-degree murder in Haag's death. He appeared in court Tuesday with a helmet on his head. If convicted, he could face up to 99 years in prison, The Chicago Tribune reported.
Lopez said her anger over the loss of her daughter will never go away.
“(Suicide) affects a whole bunch of different families -- whoever’s around that person. Just get help,” she said.
According to Fayette, Sibson’s blood-alcohol level was three times the legal limit at the time of the shooting.