Mr Finucane was fatally shot by loyalists who used sledgehammers to break down the door to his north Belfast home in February 1989.
On Wednesday, his family, who have long campaigned for an independent public inquiry, will be in London to receive the 500-page inquiry report into his death.
Sir Desmond de Silva QC was commissioned by the Government last year to undertake a legal review of the case, amid long-running allegations that the state colluded to facilitate the Belfast lawyer's murder.
The police officer in charge of the investigation 23 years ago told UTV he came to the conclusion there was state collusion in the attack.
Retired Det Supt Alan Simpson spoke to UTV's Chris Moore on the eve of the publication of the report.
He was one of the most respected detectives in the RUC and remembers arriving at the murder scene on 12 February 1989.
"I recall vividly the night of the murder," he said.
"I was at home and the phone rang and expected it was a call out. I was shocked to hear that Pat Finucane had been shot dead at his home.
"I went immediately to the scene and supervised all the forensics examinations and so on."
"The family, other than Mrs Finucane, Geraldine, she had been taken to hospital, she had been shot also in the foot, but the family had secreted themselves in the front room and had no contact with us."
At the time, the detective believed the family did not speak because they were anti-RUC, but he discovered they did not want any contact with him or other police staff due to suspected state involvement.
In the months which followed I regret to say I came round to the same conclusion as the Finucane family.
Alan Simpson, former senior RUC detective
He said that at the time he did not believe it could be possible.
"It was beyond my comprehension to think that the State would get involved in such a criminal or morally wrong act such as that."
But it eventually emerged that the British Government colluded with the UFF gunmen who killed Mr Finucane.
It became known that virtually every member of the UFF gang involved in the shooting was a Special Branch informer, including Ken Barrett, who in 2004, pleaded guilty to the murder.
Mr Simpson told UTV that in his 20-year career, he was deceived by Special Branch "many times".
"They were powerful simply because of this organisation, MI5, constantly shadowing them and guiding them in the background."
Mr Finucane's son John said for a long time "very few people believed or wanted to believe collusion was in fact, a fact."
He said: "We have a situation now where the PM accepts there was collusion in the murder of my father."
He said the family have reservations about Wednesday's report.
"We have concerns about the process and we have concerns about the independence of the man himself conducting this review. So I don't hold out much hope that tomorrow will be the final resolution for our family and if it isn't we continue to campaign."
The Finucane family say an inquiry was promised during peace talks at Weston Park - they insist it is the only resolution they can accept.
Retired Canadian judge Peter Cory, who examined allegations of collusion surrounding the Finucane case and other controversial killings at the request of the British and Irish Governments in 2001, recommended a public inquiry into the death.