By Laura Smith-Spark and Atika Shubert
ANTRIM, Northern Ireland (CNN) -- Police continued to question Sinn Fein leader Gerry Adams Friday in connection with the 1972 abduction and killing of a mother of 10 by the Irish Republican Army.
Under UK law, the police can only hold Adams without charge until Friday evening local time, unless they go to a court to request more time.
Adams, who surrendered himself for questioning at Antrim police station Wednesday evening, has long denied having any role in the death of Jean McConville, a widow who was reportedly killed by the IRA because the group believed she was a spy for the British army.
Martin McGuinness, a Sinn Fein member and the Deputy First Minister of Northern Ireland, told reporters in Belfast on Thursday that Adams' arrest was unnecessary, unjustified and politically motivated.
He said he had seen the "dark side" of Northern Ireland policing "flex its muscles in the course of the past couple of days" and that the arrest was a "'deliberate attempt to influence the elections that are due to take place in three weeks' time."
McGuinness said he was confident that Adams would be able to rejoin election campaigning shortly and would "continue to lead our party in a very positive way."
The head of the Police Service of Northern Ireland insisted the police would "conduct a thorough and professional investigation into the murder of Jean McConville," according to a statement from the force Thursday.
"This will be subject to the full rigors of scrutiny provided in the criminal justice system," the statement said.
Jean McConville's family welcomed news of Adams' arrest.
Her daughter, Helen McKendry, told CNN, "I can only do like everyone else in my case and hope that he will be brought to a court of law and be charged with my mother's murder, so my family can get truth and justice for my mother."
Adams rejects allegations
In a statement released shortly before Adams surrendered himself for questioning Wednesday, the 65-year-old vehemently denied any involvement in the killing.
"I believe that the killing of Jean McConville and the secret burial of her body was wrong and a grievous injustice," Adams said in the statement posted on his party website. "Malicious allegations have been made against me. I reject these."
The questioning of Adams was not unexpected. Adams said he told authorities last month that he was willing to meet with investigators.
"While I have never disassociated myself from the IRA and I never will, I am innocent in the abduction, killing or burial of Mrs. McConville," Adams said.
Long associated with the IRA, once considered the armed wing of Sinn Fein, Adams is a prominent Catholic politician who helped broker peace in Northern Ireland. Today, Sinn Fein is Ireland's second-largest opposition party.
A number of other people have been arrested and questioned in connection with the investigation into McConville's abduction and killing.
One man has been charged with aiding and abetting the crime. He denies wrongdoing.
The IRA admitted in 1999 to killing a number of people who have become known as "The Disappeared" -- those who vanished during the so-called Troubles, a 30-year conflict between Protestant loyalists who wanted to stay part of the United Kingdom and and Catholic nationalists who wanted to see the north united with Ireland.
Among the victims was McConville, 37, whose remains were found partially buried on a beach in County Louth in 2003. She died of a single gunshot wound to the back of the head.
CNN's Atika Shubert reported from Antrim and Laura Smith-Spark wrote and reported from London. CNN's Chelsea J. Carter and journalist Peter Taggart contributed to this report.
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