* Former head of IRA political wing denies involvement in killing
* Killing of mother of 10 among most controversial of conflict
* Adams head of Ireland's second largest opposition party
* Says concerned about questioning during EU elections
* Arrest raises legal questions (Adds White House comment, paragraph 25)
By Padraic Halpin and Neil Maidment
ANTRIM, Northern Ireland/DUBLIN, May 1 (Reuters) - Northern Irish police questioned Sinn Fein leader Gerry Adams on Thursday after arresting him under an investigation into one of the province's most notorious murders, a move that stirred fierce political reaction in Britain and Ireland.
Reviled by many as the spokesman for the Irish Republican Army in the 1980s during its campaign against British rule, Adams reinvented himself as a Northern Ireland peacemaker and then as a populist opposition parliamentarian in the Irish Republic.
His Sinn Fein party, which shares power in the Northern Ireland government, said he was arrested in the town of Antrim on Wednesday evening by police investigating the 1972 abduction and murder of Jean McConville, a widowed 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 detained 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.
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.
McConville, who was dragged away in front of her children, was one of 15 people living in strongly republican areas who were spirited away by the IRA and dumped in unmarked graves.
Her body was found only in 2003 by a man walking on a beach over the border in County Louth, which Adams now represents in the Irish parliament. The IRA accused McConville of being an informer for the British, an allegation her family has always denied.
The investigation into her 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.
Northern Ireland police took legal steps to get hold of the interviews, parts of which were published after an IRA interviewee died. The BBC played excerpts from an interview given by late IRA commander Brendan Hughes, who fell out with Adams, accusing the Sinn Fein leader of responsibility for McConville's killing.
In his book "Voices from the Grave", Boston College researcher Ed Moloney quoted Hughes as saying that McConville was killed by a squad "established by and ultimately ... responsible to Gerry Adams", a charge Adams has always vigorously denied.
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 voluntarily 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 prearranged questioning, as I understand, he probably didn't need to be arrested," Rice said.
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.
No one has been charged with McConville's murder, although Ivor Bell, 77, an IRA man in the 1970s, has been charged with aiding and abetting murder.
Boston College said on Thursday it 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 this month, and the "dark side" within the police was to blame.
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".
In Washington, White House National Security Council spokeswoman Laura Lucas Magnuson said: "On this particular issue, we understand there is an open criminal investigation under way in Northern Ireland. We refer you to the Police Service of Northern Ireland for additional details. More broadly, we encourage all parties to keep working to build a sustainable peace."
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 mother being taken from their 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 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 a memorial service for Nelson Mandela last year.
HAUNTED BY KILLING
But the killing of McConville has haunted him and has been raised repeatedly 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, or in the Irish Republic where Adams leads the second largest opposition party.
Theresa Reidy, a politics lecturer at University College Cork, said suggestions that Sinn Fein would suffer politically as a result of Adams's arrest might be wide of the mark because suspicions about the case had been around for years.
"I don't think this is a terribly new or big piece of information and the idea that it is going to significantly affect Sinn Fein in the opinion polls in the elections, I think is probably overplaying it," she said.
Adams has recently had to deal with controversy closer to home when he was forced to distance himself from his brother.
Liam Adams was sentenced last year to 16 years in prison for raping his own daughter when she was a child. Prosecutors decided not to prosecute Gerry Adams over allegations of withholding information from the police on the issue. (Additional reporting by Maurice O'Neill, Conor Humphries, Ross Kerber and Steve Holland,; Editing by Giles Elgood, David Stamp, Peter Cooney and Ken Wills)
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