Fresh doubt has been cast over Sinn Féin's claim that senior members of the party did not know about the 'coaching' of Nama inquiry witness Jamie Bryson.
North Antrim MLA Daithí McKay resigned after claims that he and party colleague Thomas O'Hara had advised the loyalist blogger before his appearance at the Stormont inquiry that Mr McKay chaired.
The advice centred on how Mr Bryson could use parliamentary privilege to name the former first minister and DUP leader Peter Robinson in front of the committee during the Assembly's investigation into the £1.3bn (¤1.5bn) sale of Nama's Northern Ireland portfolio.
Mr Robinson has strongly rejected all accusations of any wrongdoing.
Referring to the allegations of coaching, Ulster Unionist leader Mike Nesbitt said that it had been "the consistent strategy of Sinn Féin to start with flat denials before the truth is revealed about an incident".
He also called for a "credible investigation" and for "a review of policy and practice around Stormont committees".
Mr Nesbitt said "history slaps down" Martin McGuinness's insistence that nobody in the party hierarchy knew about Mr McKay's actions.
He added: "For a political party, Sinn Féin's track record is remarkably consistent, to the point that it is clearly a strategy to start with a flat denial of involvement or prior knowledge of an event like the coaching of Jamie Bryson.
"The Deputy First Minister's words ring hollow and what we need is a credible investigation.
"Never mind Martin McGuinness telling us he has spoken to the 'right people'."
Mr McKay resigned after leaked tweets showed communications between him, Mr O'Hara and Mr Bryson before the latter testified.
Mr Bryson has made allegations relating to £7m in an offshore bank account linked to the Nama deal being earmarked for a senior politician.