Former Colorado prosecutor Karen Steinhauser said the ruling was very unusual and likely means the judge believes the case is an exception to the state's shield law protecting journalists from revealing their sources.
In Colorado, journalists can be ordered to reveal their sources if they have information directly related to court proceedings, if the information can't be obtained by other means, and if the interests of the party seeking the information — in this case, the defense — outweigh the interests of the journalist. If they refuse, they can be held in contempt and jailed until they reveal the source.
Steinhauser said the identity of the source and the notebook touch on several important issues in Holmes' prosecution, including whether any potential law enforcement witnesses have committed perjury, Holmes' right to a fair trial and the possible violation of the judge's gag order.
Prosecutors didn't take a position on having Winter testify but objected to characterizations that information was leaked. Holmes' attorneys are seeking sanctions on prosecutors for information that they say was leaked.
Associated Press writer Colleen Slevin contributed to this report.