Case, a 23-year-old Kansas City man, winced as a judge read the verdict, but said nothing.
Jurors deliberated less than four hours before finding Case guilty of first-degree murder and armed criminal action. He faces a life sentence without parole.
The verdicts resolve a 1997 murder that, according to one prosecution witness, was committed by Case because he wanted to experience killing someone.
That witness, Kelly Moffett of Lenexa, was Case's girlfriend on Oct. 22, 1997. She testified that she was with Case when he killed Anastasia WitbolsFeugen, 18, of Independence.
Moffett, now 19, testified that she had lied to police about the murder until the secret drove her to depression and drug addiction. She said she felt compelled two years ago to tell the truth.
Jackson County Prosecutor Bob Beaird said after the verdict was announced that Moffett was the only reason Case was brought to justice.
"In the end," Beaird said, "Kelly Moffett came forward and did the right thing -- she had a good heart."
Bob WitbolsFeugen, the victim's father, said he understood that Moffett was 14 at the time of the murder, vulnerable and controlled by Case.
"She had to endure a great deal of personal pain...before she could tell the truth," he said.
Nadine Case of Gladstone, the defendant's grandmother, said the verdict was disappointing because it was not based on physical evidence.
"It was just one person's word against another,' she said.
Moffett told police in 1997 the same story as Case. They were in a car with their friends, WitbolsFeugen and Justin Bruton, 20, of Kansas City. They said the victim and Bruton argued and they last saw WitbolsFeugen when the angry teen got out of the car at Truman Road and Interstate 435, near Lincoln Cemetery, where her body was discovered.
Bruton, the victim's boyfriend, committed suicide after the murder.
At trial, Moffett testified that the story in 1997 was a lie. She said Bruton and Case told her that they were going to kill WitbolsFeugen for nagging Bruton about the lack of commitment in their relationship.
Bruton told her he couldn't kill WitbolsFeugen, but Case, who was fascinated with death, would kill her. Moffett testified that Case also told her he wanted to see what it was like to kill someone.
While Bruton and WitbolsFeugen talked about their relationship at the cemetery, Moffett said, Case got out of the car and shot WitbolsFeugen. Bruton tried to stop him, Moffett said, but it was too late.
Case testified this week that the original story that WitbolsFeugen got out of the car was true, that Moffett later lied because he spurned her.
In closing arguments Thursday, lawyers clashed over who was telling the truth.
Theresa Crayon, assistant prosecutor, told jurors that Moffett had no reason to falsely accuse Case.
"Kelly gets nothing out of this other...than the chance to say, `I lied to the police. I lied for a long time. I'm a drug addict. I'm an alcoholic. I have been a mess.' "
Moffett dealt with her lie by drinking and using drugs, Crayon said, and Bruton dealt with it by killing himself.
She noted that WitbolsFeugen died suddenly without any signs of an assault or robbery -- "killed by somebody she knew."
Crayon told jurors to listen again to tapes Moffett made for police after she came forward -- two calls in which she tried to get Case to admit to the killing.
He never admitted it, nor did he deny it.
Crayon read the transcript of what Moffett told Case: "If you could seriously explain to me as to why you actually felt the need to kill her, then that would really help me feel better...I mean, was there seriously any reason to all of this?"
Crayon read Case's reply: "We shouldn't talk about this."
Defense lawyer Horton Lance told jurors the tapes proved nothing. His client was ill when he spoke to Moffett, he said, and "if somebody says something crazy, you ignore it."
His client just thought it was another call from his "crazy, psycho ex-girlfriend," Lance said.
He told jurors not to believe Moffett, and called her a drug addict with a grudge who wanted sympathy and attention.
He said WitbolsFeugen died because she made herself "easy prey" by cutting through a dark graveyard.
She "was the victim of a random act of violence committed by some unknown stranger," Lance said.
Jury members declined to comment.