The New York-based Lawyers Committee for Human Rights called for an independent inquiry into the murder of Mr Finucane, who was killed by loyalist terrorists. It said that suspicions persisted about official collusion in the killing.
The 92-page report, which was drawn up following a visit by three American lawyers to Northern Ireland last year, also called for an inquiry into what it described as widespread and persistent allegations of harassment of defence lawyers by the RUC.
The RUC, in a response, branded the report yesterday as falling 'far short of the balance, fairness, depth and context which the sensitive and complex situation in Northern Ireland requires.'
The force's deputy chief constable, Michael McAtamney, said that he feared it would exacerbate difficulties and contribute to 'divisiveness and unjustifiable contention'.
The Northern Ireland Office said yesterday that the report had made extremely grave allegations about security force collusion with loyalist terrorists but had failed to produce any credible evidence of this.
The report states: 'The evidence suggests that Finucane's success as a lawyer subjected him to various forms of official intimidation prior to his murder.
'There is also additional evidence pointing to collusion between the Ulster Freedom Fighters and the security forces in the murder itself.'
The authors said they were told by sources, whom they did not name, that RUC interrogators at Belfast's Castlereagh holding centre had told UFF suspects that Mr Finucane and two other solicitors were 'the brains behind the IRA'.
The report also cites material from the Independent and a BBC Panorama programme in which Brian Nelson, an acknowledged British Army agent operating within the UFF, said he had informed his military intelligence handlers that the assassins were targeting Mr Finucane.
It quotes a senior Northern Ireland Office official as saying that those who killed Mr Finucane were known and are currently serving time for other offences.
The report says a number of solicitors, whom it names, complain that clients are regularly warned away from them by interrogating detectives. The solicitors say police regularly make remarks such as 'you must be a Provie bastard because you got one as a solicitor'.
One solicitor told the American lawyers that two of his clients, who had been involved with republican groups, had both told him that detectives said they, and he, would be killed. He said both had subsequently been shot dead by loyalists.