Where do we draw the ethical line between journalists and private investigators?

I came across an article in the Wall Street Journal titled "Did Students Pay Witnesses?"

The article is an update regarding the unrelenting battle between Northwestern's journalism students involved with Medill's Innocence Project, investigating Anthony McKinney's case and Illinois prosecutors.

"Prosecutors Allege Journalism Class Sought Testimony to Overturn Murder Conviction"

According to the Wall Street Journal,"Prosecutors said in the court filing that Mr. Drakes recanted his videotaped confession and claimed it came in exchange for $40 he was given by a cab driver hired by someone from the Northwestern team. Mr. Drakes had the cab stop near a crack house, where he spent the $40, according to the filing."

I believe exchanging money for a recanted videotaped confession is unethical on behalf of both the journalism students and witness. But I also think it is erroneous that prosecutors found Mr. Drakes crack cocaine purchase relevant to their case, since the students never advised the cab to stop at the crack house. And it is unrelated to Drakes recanted confession pertaining Anthony McKinney's lifelong prison sentence.

I agree with prosecutors on how it's unethical the "students were acting as private investigators, not journalists." I don't understand "why" investigative journalism students are investigating wrongful convictions. I think it is our obligation to ask authorities and lawyers questions to further our stories and provide the public with accurate information. Not prove whether convicted felons are guilty or innocent.

The only correlation I see between Medill's Innocence Project investigations and journalism is that wrongful convictions are newsworthy, since innocent people don't deserve to suffer indefinitely in a prison cell nor await their turn on death row, regardless we are not investigators. How do you feel?

I also think prosecutors are placing subpoena's on irrelevant information such as "the students' unpublished notes and grades, evaluations of their performance and private emails between the students and Mr. Protess." Because the ultimate goal is to prove a convicted felon Anthony McKinney, is an innocent man.

However "prosecutors alleged the students were not seeking to publish their findings, but to collect evidence exonerating the defendant." They claimed "the distinction is important because, as journalists, the information sought by prosecutors would be protected under the Illinois Reporter Act."

I think this persistent conflict raises questions regarding ethical journalism practices since it puts the case and Medill's Innocence Project into a whole new perspective.

Where should we draw the ethical boundary between practices of journalists and private investigators? Is it ethical for journalists to dedicate their job to gathering evidence to prove a person in prison or on death row, is infact innocent? Or should we only report on the investigators findings?