Former PSNI chief constable George Hamilton has claimed that Kevin McGuigan was killed with the "knowledge and concurrence" of senior republicans.
Details are revealed in a new book by veteran journalist and commentator Brian Rowan.
'Political Purgatory: The battle to save Stormont and the play for a new Ireland', has been described as a must read for anyone involved in Irish and British politics.
Including contributions from senior political figures in recent years, the book explores some of the most dramatic phases in the recent political history at Stormont examining key moments from Martin McGuinness's resignation as deputy first minister and subsequent electoral loses suffered by the DUP.
In the book it is revealed how First Minister Arlene Foster texted her Sinn Féin counterpart in is final days and Mr Hamilton provides a glimpse into his relationship with the senior republican and discusses attending his funeral.
He also refers to Mr McGuigan who was shot dead in August 2015 outside his Short Strand home in east Belfast.
Mr McGuigan is believed by some to have been involved in the shooting of prominent republican Gerard Jock Davison in the Markets area in May that year.
Mr McGuigan, who denied any involvement, had presented himself to police for questioning after Mr Davison’s murder.
Police later blamed members of the Provisional IRA and Action Against Drugs for the McGuigan killing but said there was no information to suggest it had been sanctioned or directed at a senior level in the republican movement.
Mr Hamilton, who was chief constable at the time, now appears to suggest senior republicans may have had knowledge of the killing.
"Within a short period of time, the investigation team were pursuing a strong line of enquiry that McGuigan had been murdered by members of the IRA" he wrote.
"As the investigation developed, it became clear that there were reasonable grounds to suspect that this was with the knowledge and concurrence of senior figures within the republican movement.
"There was public and private commentary of ‘told you so’ regarding my decision to attend the West Belfast Festival and engage so publicly with republicanism."
Mr Hamilton also reveals how he and Sinn Féin Deputy First Minister Martin McGuinness spoke about 'grace and generosity' before his death.
Mr Hamilton has also broken his silence on attending the ex Deputy First Minister's funeral in Derry in 2017.
He also writes that he had got to know Mr McGuinness on a personal level and would not breach the confidentiality of their private conversations.
"But, in general terms, we had spent time discussing the peace process; the challenges of maintaining support for it, the impact of policing and the need to nudge that process on to the next level of reconciliation," he wrote.
"We had discussed concepts such as ‘grace’ and ‘generosity’ and the need for those ingredients to be obvious in the words and actions of leaders across all sectors.
"On the other hand, I knew the hurt that had been caused in the past by Martin and his IRA comrades.
"In my opinion the violence was never justified and could never be justified.
"Under Martin’s leadership the mainstream republican movement had moved away from violence as a means of achieving their political aims."
'Political Purgatory: The battle to save Stormont and the play for a new Ireland' is published by Merrion Press and is available now.
