Jana Winter, a New York-based investigative journalist, was sent to Aurora, Colo., last year to cover the July theater shooting that killed 12 people and wounded 58 during a midnight screening of "The Dark Knight Rises."
While there, Winter filed a story citing anonymous law enforcement sources saying Holmes had sent a journal filled with violent threats and drawings to his psychiatrist just before carrying out the act.
Holmes' defense team contends the leak violated gag orders issued by a judge in the days after the shooting and were granted authority to begin a search to identify Winter's source.
After initial questioning of a dozen law enforcement personnel yielded nothing, Holmes' lawyers subpoenaed Winter, who said if she is called to testify she will not reveal her source and could face six months in jail for contempt of court.
Colorado District Court Judge Carlos Samour Jr. said he has not decided whether he will allow Holmes' defense team to call Winter to the witness stand.
Colorado is one of 32 states with a reporter "shield law" -- meant to prevent journalists from having to give up their sources unless the information is central to determining guilt or innocence at trial.
Winter's lawyer said the source for one story, given the huge crush of media coverage, is not central to the case. First Amendment advocates agreed, Fox News reported.
"The subpoena of Ms. Winter is absolutely troubling," said University of Colorado at Boulder journalism professor Len Ackland. "An important way that journalists inform the public -- their obligation as self-appointed, self-anointed public servants -- is to have sources tell them what is really happening on matters of public interest. Such subpoenas clearly can keep journalists from doing their work as well as intimidating potential sources."