The Chief Constable has told the Policing Board there was "absolutely no GB security services involvement before, during or after" when quizzed on the investigation into the suspected theft of documents from the Police Ombudsman.
George Hamilton admitted he was uncomfortable with arresting journalists Barry McCaffrey and Trevor Birney, who were detained by an outside force in a bid to boost public confidence.
The confidential documents concerned a police investigation into the Loughinisland massacre by loyalists of six men in 1994.
The two journalists produced No Stone Unturned, a documentary about the UVF atrocity examining claims of state collusion.
In reply to a question from Sinn Fein MLA Gerry Kelly, the Chief Constable denied any involvement by MI5 in the police investigation.
He also said he had read a statement of complaint from the Police Ombudsman's Office about the theft, after the Ombudsman had said that none was made.
"I want to bust the myth that there was no statement of complaint," he said.
"We had a report of what was quite a serious offence in terms of theft of the documents.
"I have read the statement, unequivocally, I'm clear in my own mind, 100%.
"The office of the Ombudsman reported the theft of documents on October 4, 2017. After that the documents appeared in a documentary film which we were aware of before its release. That was a definite starting point.
"It was our responsibility to investigate, once we had reasonable cause to believe an offence had been committed.
"It is not a comfortable place to arrest journalists, but we have an obligation under the law.
"We as a police force were aware of the challenges attached.
"Sometimes these things are difficult and sensitive and we are not looking to discredit journalists, but our actions have been lawful and proportionate."
Mr Hamilton said he had asked Durham Constabulary to carry out the investigation in a bid to boost public confidence, and said the Durham Chief Constable Michael Barton had indicated a willingness to answer questions about the arrests at a future meeting of the Policing Board.