The surprising twist came Friday, after defense attorneys spent a day calling witnesses to build an alibi for the defendant, 26-year-old Willie Clark. His attorneys are hoping to switch the focus away from him to an alternative suspect.
Denver prosecutors said the man who refused to testify on their behalf Thursday is important because he is expected to say Clark admitted to killing Williams in a drive-by shooting on New Year's Day 2007.
The witness, who has said in court that he fears for his life if his name became public, is scheduled to testify when prosecutors present their rebuttal to the defense's case sometime next week.
Two other witnesses have also been jailed for refusing to testify in the trial, which has featured tight security and a ban on cell-phones and other electronic devices inside the courtroom.
Clark, a suspected gang member, is also expected to decide next week whether he'll take the stand in his own defense.
"I have to confess," Denver district Judge Christina Habas told Clark on Friday. "I've given you a lot of time to think about this."
Clark responded that he's undecided, adding that he wants to wait until all of the witnesses testify in his defense.
Clark faces first-degree murder and other charges. Clark maintains he is innocent.
Williams was in a Hummer limousine with other football players when he was shot. They had just left a Denver nightclub where prosecutors say Williams' group had gotten into an altercation with a group that included Clark.
The defense on Friday called on security guards to testify about what they saw as people were leaving the nightclub where Williams had been.
