The senior policeman leading the investigation into the murder of Pat Finucane has claimed that more new information has been uncovered.
Sir John Stevens is expected to deliver a report into the 1989 murder of the north Belfast solicitor later this year.
Mr Finucane was shot dead by UFF gunmen in front of his wife and three children on February 12 1989.
It subsequently emerged that UDA double agent Brian Nelson had been heavily involved in the plot to kill Mr Finucane even though he was a member of the British army's highly secretive Force Research Unit.
Nelson was later sentenced to 10 years in jail for his involvement in UFF murder bids, but this did not include the murder of Mr Finucane.
Since 1990 Sir John Stevens has led three investigations into allegations of security force collusion in the murder of Mr Finucane and other nationalists.
'Stevens III' began in 1999 and after long delays is finally expected to be completed in April. Sir John yesterday justified the delay by claiming that "massive" amounts of new documents relating to the case had only just been discovered.
In an interview with The Guardian newspaper yesterday Sir John de-fended the delay in publishing his report claiming that the Finucane investigation was the "longest and most complex criminal investigation" ever undertaken in Britain.
"I think this inquiry has been the most difficult thing I have ever done in my life, and I've been around for a while," he told the paper.
"I think I could say that some of the things that we found have surprised me.
"I've been in policing for too long to be shocked by very much, quite frankly.
"But, I can assure you that we have got to the very heart of what's gone on, and its taken a long time to do."
Suggesting that a significant amount of new information had only recently come to light, Sir John said: "We are pursuing the Finucane murder vigorously. We have interviewed several members of the army.
"We recently interviewed a brigadier for a lengthy period of time. There are other leads coming in now that we never knew about."
And the Metropolitan police chief admited that his inquiry has been hampered by the fact that he is investigating the state's own security agencies.
Sir John said that the inquiry had taken so long because of "the secrecy and the fact that we are investigating not just the army and what was the RUC but MI5 activity as well".
"It goes to the very heart of what takes place in terms of intelligence gathering in this country," he said.
Suggesting that the events surrounding Pat Finucane's murder were akin to a spy novel, Sir John stated: "Various eminent people have been to look at the inquiry.
"And I'll just say one thing that they have said – if you read what has happened in this inquiry in a book, you would not believe it. And I would confirm that."
When contacted yesterday a spokeswoman for the 'Stevens III' refused to identify the "eminent people" who had viewed Sir John's inquiry or the detail of the "new evidence" which has been uncovered.
