* Former head of IRA political wing denies involvement in killing
* Killing of mother of 10 among most controversial of 'Troubles'
* Adams head of Ireland's second largest opposition party
* Says concerned about questioning during EU elections
* Arrest raises legal questions
Reviled by many in Britain as the spokesman for the Irish Republican Army in the 1980s, Adams reinvented himself as a Northern Ireland peacemaker and then as a populist opposition politician in the Irish parliament.
His Sinn Fein party said he was arrested on Wednesday evening by police investigating the 1972 abduction and murder of Jean McConville, a mother of 10 children.
Adams can be held for up to 48 hours, or until 8 p.m. (1900 GMT) on Friday, before a judge must rule on whether he can be held any longer. Under British anti-terrorism laws, a suspect can be held for up to 28 days before being charged.
Adams, 65, who has always denied membership of the IRA, said he was "innocent of any part" in the killing, which he said was "wrong and a grievous injustice to her and her family".
"Well publicised, malicious allegations have been made against me. I reject these," he said in a statement.
The investigation into McConville's killing has been revived by the release of a series of taped interviews given by former guerrillas from the Northern Ireland conflict for a research project at Boston College in the United States.
The Northern Ireland police took legal steps to acquire the interviews, parts of which have already been released after one IRA interviewee died. Participants had been told by the college that their words would be released only after they died.
Joe Rice, a Belfast lawyer who has represented senior Republicans over the past three decades, said the tapes would be of little evidential value but would offer a lot of "probative value", meaning police could play them and demand that Adams respond to details in the recordings.
Rice said Adams had presented himself voluntarilty for police questioning and would not have expected to be arrested.
"It looks like his legal team are surprised that he was detained overnight. I would have thought that if it was to be a pre-arranged questioning, as I understand, he probably didn't need to be arrested," Rice said.
Nuala O'Loan, a former Northern Ireland police ombudsman who investigated the police's handling of the murder in 2006, was asked if the Boston tapes would be admissible in court.
"I hesitate to comment as I have not seen these tapes but the indicators are that these are useful as investigative tools but I would not go further than that," she told the BBC.
Another Belfast lawyer, who did not want to be named, added: "The main problem with historic crimes is that it doesn't offer the defendant an opportunity to provide an alibi, simply because in the passage of time your memory fades."
There would be no means for Adams to cross examine the person who provided the recordings, so he could not attack the person's credibility, the lawyer said.
Under the 1998 Good Friday agreement, which drew a line under 30 years of sectarian strife in the British province, those convicted of paramilitary murders during the conflict would have life sentences reduced to two years.
Boston College officials said on Thursday they had not been involved in the Northern Irish authorities' actions.
"We are not privy to the actions of British law enforcement and have had no involvement in the matter since the U.S. court issued the order to remand portions of the archived interviews last year," said Jack Dunn, a spokesman for the school. "As a result, it would be inappropriate to comment on this issue."
Northern Ireland's deputy first minister, former IRA commander Martin McGuinness, said Adams' arrest was a deliberate attempt to influence European and local elections on May 23, and the "dark side" within Northern Ireland policing was to blame.
O'Loan disagreed. "I am quite sure the police would not have arrested Mr Adams had they not felt that they had the reasons to do so."
Political leaders on both sides of the Irish Sea rejected Sinn Fein's suggestions of a political motive for the arrest.
British Prime Minister David Cameron said there was absolutely no political interference, a view echoed by Peter Robinson, First Minister of Northern Ireland.
The chairman of Irish Prime Minister Enda Kenny's party, Fine Gael, said Sinn Fein's suggestions of a political motivation were "baseless and desperate".
McConville's body was found in 2003 by a man walking on a beach in County Louth, which Adams now represents in Ireland's parliament. The IRA accused McConville of being an informer for the British, a charge her family has always denied.
McConville, a Protestant who had married a Catholic, was said to have gone to the aid of a wounded British soldier serving in the province, which was torn by violence between Catholic republicans and pro-British Protestants.
Michael McConville, who was 11 when he witnessed his widowed mother being taken from their Belfast home, said the IRA had warned the family not to go to the police.
"The IRA stopped me from doing it," he told Irish broadcaster RTE. "The IRA said it would kill me or some of my family members."
As head of the political wing of the IRA, Sinn Fein, Adams was a pariah in 1980s Britain, banned from speaking on British airwaves, forcing television stations to dub his voice with that of an actor.
Former Prime Minister John Major once said the thought of sitting down with him "turned his stomach". Adams emerged from the political cold in October 1997 when he shook hands with Labour Prime Minister Tony Blair at their first meeting.
A year later Adams helped broker a peace deal that largely ended the violence between Catholic militants seeking union with Ireland and mainly Protestant militants, who wanted to maintain Northern Ireland's position as a part of Britain.
Since that peace deal Adam's role as a statesman has grown. He is a regular visitor to the White House and was a guest of honour at the funeral of former South African president and anti-apartheid hero Nelson Mandela last year.
HAUNTED BY KILLING
But the killing of McConville has haunted him and has been repeatedly raised in interviews during his career as a member of the Irish parliament.
It is unclear what effect the arrest might have on Northern Ireland's power-sharing government, but the arrest will have major ramifications in the Republic of Ireland where Adams leads the second largest opposition party.
He was forced to distance himself from his brother. Liam Adams was sentenced last year to 16 years in prison for raping his daughter when she was a child. The Public Prosecution Service decided not to prosecute Gerry over allegations of withholding information from the police on the issue.