Michael McConville, whose mother was killed by the IRA, has told police he will not identify the men who dragged his screaming mother from her home more than 40 years ago.
Jean McConville disappeared from west Belfast in 1972. Mr Adams was arrested at Antrim police station last night.
Mr McConville said he recognised local faces when the gang arrived to drag his mother away screaming in terror, but he said if he told police he would be an informer.
"Everybody thinks that the IRA has gone away but they have not. If we tell we will be shot."
He noted that suspected dissident republicans opposed to the peace process shot a man dead in west Belfast last week.
Commenting on the perceived threat to Mr McConville, Northern Ireland First Minister Peter Robinson said: "We have had a lot of words from Sinn Fein about how wrong what happened to Mrs McConville was.
"Here is an opportunity. This is the same republican movement of which they are a part, that the people who carried out this act were a part.
"Let them meet Michael McConville, let Michael McConville impart to them the names of those who were involved.
"I challenge Sinn Fein that they should do their civic duty and responsibility of handing over those names to the PSNI.
Deputy First Minister Martin McGuinness blamed the "dark side" of policing for his leader's detention during the election campaign and insisted Mr Adams could have been questioned without being arrested.
He said the questioning centred on books Mr Adams had written and what others said about him - those "maliciously and vehemently" hostile to the peace process.
Mr McGuinness claimed: "People who could be described as former republicans (are) targeting the Sinn Fein peace strategy and targeting the leader of Sinn Fein, Gerry Adams.
"It has been disappointing to see the efforts of some of those people together in consort with the dark side within policing."
He drew a contrast between "reformers" within the police service, whom he supported, and those from the "dark side" responsible for the arrest of his friend Mr Adams.
"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.
"I view his arrest as a deliberate attempt to influence the outcome of the elections that are due to take place in three weeks' time, north and south on this island.
"That raises very serious questions around why that is the case and what is the agenda."
Dodds speaks in House of Commons
Democratic Unionist MP Nigel Dodds said victims need to be reassured that people will be brought to account "no matter their status or their standing in society".
During business questions in the House of Commons, Mr Dodds asked Commons Leader Andrew Lansley: "In light of the arrest and detention of a certain Gerry Adams in connection with the murder of Jean McConville, a mother-of-10 who was abducted and murdered by the IRA - of which Gerry Adams was commander in Belfast in the 70s - simply for going to the aid of a dying soldier, would you agree it's an appropriate time for a debate on victims, so that they can be reassured that evidence will be followed and people will be brought to book no matter their status or their standing in society?"
Mr Lansley replied: "You will I'm sure understand that I'm not in a position to comment at all on any ongoing police investigation.
"But the point you make about victims is an important one.
"I think we should always, indeed as I know (Northern Ireland Secretary Theresa Villiers) at this despatch box recently has made very clear in relation to wider circumstances about the questions on the on-the-run terrorist review that we will always make sure the needs of justice are served, and victims can see that we are continuing to pursue the issues relating to seeing justice served."
Kenny denies Adams arrest was politically motivated
Republic of Ireland Taoiseach Enda Kenny rejected suggestions from senior Sinn Fein figures that the arrest of Mr Adams had been politically motivated.
"What is the most important thing here? The most important fact is that Jean McConville was murdered, a widowed mother-of-10 children, and her body was not found for very many years," Mr Kenny said.
"This is still a live murder case, this is still a live investigation.
"All I can say is that I hope the president of Sinn Fein answers in the best way he can, to the fullest extent that he can, questions that are being asked about a live murder investigation."
Meanwhile, another senior figure in Ireland's coalition government, Labour deputy leader Joan Burton, said the murder of Mrs McConville was a war crime for which all people involved should be brought to justice.
"If what happened to Jean McConville and her family had happened in any other country it would be treated properly as a war crime," she said.
Ms Burton said Mrs McConville was executed and her body treated like a dog.
"Gerry Adams just will not disassociate himself from the organisation that did that," she said.
She added that certain standards in relation to war crimes have to be acknowledged and addressed.
Mr Adams wrote to the PSNI on March 23 to say he was available and willing to help with their inquiries into Ms McConville's murder.
Mary Lou McDonald hits out at unionist 'Old Guard'
Sinn Fein deputy leader Mary Lou McDonald said detectives had waited more than a month to take him up on the offer because of a "politically motivated" attempt to undermine the party ahead of local and European elections on both sides of the Irish border.
She claimed that reactionary figures within the Democratic Unionist Party as well as the minority hard-line Traditional Unionist Voice and "old guard" elements within the PSNI created pressure to choreograph the timing of the arrest.
"My own view is that those two things have coalesced for the timing of this to fall right in the middle of an election campaign," Ms McDonald added.
Mr Adams has vehemently rejected allegations made by former republican colleagues that he had a role in ordering the death of mother-of-10 Mrs McConville - who was wrongly accused of passing information to the British army in Belfast.
The former West Belfast MP was detained last night after voluntarily presenting himself for interview at a police station in Antrim.
No-one has ever been charged with the murder of Mrs McConville. But after years without progress in the criminal investigation there have been a series of arrests in recent weeks.
A veteran republican - 77-year-old Ivor Bell - was charged in March with aiding and abetting the murder. Five other people have been detained and questioned.
The recent police activity followed a decision by a US court compelling a Boston university to hand over to the PSNI recorded interviews with republicans about Mrs McConville's murder.
Boston College interviewed a number of former paramilitaries about the Troubles on the understanding transcripts would not be published until after their deaths - but that undertaking was rendered ineffective when the court last year ordered that tapes that contained claims about the killing be given to detectives.
Mr Adams, 65, has always denied IRA membership or any role in Mrs McConville's death and said in March he would be available to meet with detectives if they wished to speak with him.
He stepped down as MP and is a representative for Co Louth in the Irish Dail.
The veteran republican presented himself at Antrim police station by prior arrangement with officers.
He issued a statement minutes after the PSNI announced an arrest had been made.
"While I have concerns about the timing, I am voluntarily meeting with the PSNI this evening," he said last night, questioning why police chose to interview him in the run up to an election.
"As a republican leader I have never shirked my responsibility to build the peace. This includes dealing with the difficult issue of victims and their families. Insofar as it is possible I have worked to bring closure to victims and their families who have contacted me. Even though they may not agree, this includes the family of Jean McConville.
"I believe that the killing of Jean McConville and the secret burial of her body was wrong and a grievous injustice to her and her family.
"Well publicised, malicious allegations have been made against me. I reject these.
"While I have never disassociated myself from the IRA and I never will, I am innocent of any part in the abduction, killing or burial of Mrs McConville."
Mrs McConville, a 37-year-old widow, was dragged away from her home in the Divis flats, west Belfast, by an IRA gang of up to 12 men and women after being accused of passing information to the British army in the city.
She was murdered and secretly buried, becoming one of the so-called disappeared victims of the Troubles.
An investigation later carried out by the Northern Ireland Police Ombudsman rejected the claims that she was an informer.
She was shot in the back of the head and buried 50 miles from her home.
Clearly embarrassed by the killing, the IRA did not officially admit responsibility for the murder until 1999 when information was passed to police in the Irish Republic.
It was not until August 2003 that her remains were found on Shelling Hill beach, Co Louth.