Sinn Féin leader Gerry Adams has criticised the timing of his arrest over the IRA murder of Jean McConville.
Speaking after his release following four days of questioning, Mr Adams said there was a "sustained, malicious, untruthful campaign" alleging his involvement in the 1972 killing.
A file will be sent to the Public Prosecution Service, police said as he was released.
Mrs McConville's son said the family's fight for justice would continue.
Jean McConville, a 37-year-old widow, was abducted from her Belfast home, shot and secretly buried.
Her body was found on a beach in County Louth in 2003.
"The McConville family is going to stay to the bitter end until we get justice," Michael McConville said.
"These have been quite difficult days for the McConville family and they have been very stressful. We would like the people to know that the family is going to stay to the bitter end until we get justice for our mother.
"We know it is going to be a long road, but we have already been fighting for justice for 40 odd years and we are not going to stop now."
Speaking at a press conference in Belfast following his release on Sunday evening, Mr Adams said he had contacted Northern Ireland police two months ago about the McConville case.
He said he had gone voluntarily to police last week and then been arrested. He questioned their timing in the middle of an election campaign and claimed they could have used discretion rather than arresting him. They did not need to use "pernicious, coercive legislation to deal with a legacy issue", he said.
The Sinn Féin leader said police had conducted 33 taped interviews and detectives had presented him with old photographs of himself and Deputy First Minister Martin McGuinness and with interviews conducted by people who were "enemies of the peace process".
He added he did not go to Antrim police station "expecting special treatment", but said his arrest had sent out the "wrong signal".
Mr Adams, a former MP for West Belfast and now an elected representative for County Louth in the Irish parliament, presented himself for interview by prior arrangement with detectives on Wednesday night. He was then arrested under the terrorism act for questioning about the IRA murder of mother of 10 Mrs McConville.
At Sunday's press conference, he again said he was innocent of any involvement in her murder.
He said Sinn Féin remained fully committed to the political process in Northern Ireland. "The IRA is gone, it's finished," he added.
"I want to make it clear that I support the PSNI."
Prime Minister David Cameron discussed Mr Adams' release on Sunday night during a phone call with Irish Prime Minister Enda Kenny.
On Sunday, as loyalists blocked the front of the police station, Mr Adams was taken out by a different exit.
Later on Sunday night there were reports of disorder in the loyalist Sandy Row area of south Belfast.
Other political parties have rejected claims that the timing of the arrest was political.
Democratic Unionist Jeffrey Donaldson said: "There is nothing within the law of Northern Ireland or the UK that says that during an election campaign the police have to suspend the investigation of murder.
"That would be absurd and would be political policing."
Ulster Unionist leader Mike Nesbitt said: "Our thoughts are with the McConville family tonight. Their 42 years wait for truth and justice makes Gerry Adams's complaint over the 'two month' delay between him offering to speak to the PSNI and their response seem churlish to say the least.
"Let us remember it was Gerry Adams who initiated this sequence of events."
Social Democratic and Labour Party leader Alasdair McDonnell said: "I see the fact that Gerry Adams presented himself voluntarily to a police station. As the timing was his, I find it hard to justify when there would be an appropriate time to arrest a political leader."
Northern Ireland Justice Minister David Ford of Alliance said he saw politicians from a variety of backgrounds seeking to interfere in policing.
"I think politicians would be extremely well advised to steer out of allegations of political policing," he said.
The Traditional Unionist Voice party said: "The release of Adams without charge raises serious questions as to how far the authorities blinked in the face of Sinn Fein threats and tantrums."
Lawyers to decide
The decision to release Mr Adams means that prosecution lawyers will decide if charges will be brought.
The file sent by the police to the Public Prosecution Service (PPS) will detail the evidence gathered.
It will be up to PPS lawyers to decide if there is enough evidence to bring any charges and what those charges would be.
The test for prosecution is met if there is sufficient evidence that can be admitted in court to provide a reasonable prospect of conviction. Lawyers must also decide if the prosecution is required in the public interest.
It could take some time for police to prepare the file for the PPS, with prosecutors then taking a further period to assess whatever evidence is presented.
Northern Ireland's director of public prosecutions, Barra McGrory QC, is a former solicitor for Mr Adams and so will delegate a decision on the file to his deputy.
Mrs McConville is one of Northern Ireland's Disappeared, those who were abducted, murdered and buried in secret by republicans during the Troubles.
She was kidnapped from her home in Divis Flats in west Belfast in front of her children after being wrongly accused of being an informer to the British Army.
Boston College tapes
Last month, Ivor Bell, 77, a leader in the Provisional IRA in the 1970s, was charged with aiding and abetting the murder, and there have also been a number of other arrests recently.
The case against Mr Bell is based on an interview he allegedly gave to researchers at Boston College in the US.
The Boston College tapes are a series of candid, confessional interviews with former loyalist and republican paramilitaries, designed to be an oral history of the Troubles.