Northern Ireland Chief Constable Matt Baggott has said the investigation involving the arrest of Gerry Adams will be "effective, objective and methodical".
The Sinn Féin leader remains police custody tonight as he continues to be questioned about the murder of a Belfast mother-of-10 Jean McConville. He denies any involvement in the crime.
The former West Belfast MP and current Louth TD can be held for up to 48 hours without charge, with officers having an option to apply to a judge for that detention period to be extended if they think it is required.
When asked this evening about the McConville probe in the context of the legacy of Northern Ireland’s troubled past, Mr Baggott said: “Effective investigation applies to any unsolved matter and it would be inappropriate for me to comment on any individual investigation other than to say they will be objective and methodical.”
The PSNI later issued a further statement on the back of Mr Baggott's comments at the Policing Board.
A PSNI spokesman said: “Today at the Policing Board the Chief Constable emphasised that the police will conduct a thorough and professional investigation into the murder of Jean McConville.
“This will be subject to the full rigours of scrutiny provided in the criminal justice system. Police have a duty to impartially investigate serious crime including murder.
“It is the police’s duty to make relevant enquiries, interview those with information, arrest and question suspects and, in consultation with the PPS (Public Prosecution Service), to either charge or submit a file to the PPS in relation to the investigation. This procedure is being followed in this case.
“As one individual has been charged with serious offences and files are being prepared in relation to other individuals, it would be inappropriate to comment further other than to reiterate the Police Service’s commitment to treat everyone equally before the law.”
Earlier today, Northern Ireland's Deputy First Minister blamed the "dark side" of policing conspiring with enemies of the peace process for the arrest of Gerry Adams.
Martin McGuinness acknowledged that Jean McConville was the victim of a terrible wrong done by the IRA but said Wednesday's detention was a deliberate attempt to influence the outcome of European elections due in three weeks' time.
The head of Sinn Féin's team at the devolved Northern Ireland Assembly has strongly condemned the shooting dead of police officers by dissident republicans opposed to the peace process.
But the arrest of Mr Adams in the midst of an election campaign has angered republicans who hope to make major vote gains in the Republic.
Mr McGuinness said: "I think we have seen that dark side flex its muscles in the last couple of days."
His partner at the head of the Stormont coalition, First Minister Peter Robinson, said it would have been political policing had the PSNI decided not to investigate Mr Adams.
The Democratic Unionist Party (DUP) leader said: "It strengthens our political process in Northern Ireland for people to know that no one is above the law, everyone is equal under the law and everyone is equally subject to the law."
Mr McGuinness said serious questions had to be asked of the agenda of those behind the arrest.
"In the mouth of an election the leader of a political party experiencing huge growth all around the island finds himself under arrest."
He claimed Sinn Fein had been told by "senior" and "reforming" figures within the Police Service of Northern Ireland (PSNI) that there was still a dark side within policing.
He said some former republicans who were "maliciously and vehemently" hostile to the peace process had been targeting Mr Adams.
"It is quite disappointing to see the efforts of those people now in consort with the dark side within policing."
Mr McGuinness said his colleague played a pivotal role in bringing peace to Northern Ireland.
"For over 20 years we have worked very, very closely in developing the peace process, bringing about the political and security transformation that the public enjoy today and, in my opinion, in the course of supporting the peace process, he has been the single most influential figure in the process," he said.