BELFAST — Northern Ireland police are casting a wider net in their efforts to prove that Sinn Fein leader Gerry Adams once commanded the outlawed Irish Republican Army and ordered the 1972 killing of a Belfast mother of 10, according to party colleagues and retired militants.
Details of an expanding trawl for evidence emerged Saturday as detectives spent a fourth day questioning Adams about the IRA’s abduction, killing, and secret burial of Jean McConville 42 years ago — an investigation that has infuriated his IRA-linked party.
Adams had been scheduled to be charged or released by Friday night but a judge granted police a 48-hour extension of his detention. Adams, 65, took part in the court hearing via a video link from the police interrogation center west of Belfast.
Sinn Fein’s deputy leader, Martin McGuinness, said he had been told by Adams’s legal team that detectives were questioning him about many of his speeches, writings, and public appearances going back to the 1970s, when he was interned without trial as an IRA suspect and wrote a newspaper column from prison using the pen name ‘‘Brownie.’’
Sinn Fein has said most of the detectives’ questions being posed to Adams concerned allegations made by IRA veterans in a Boston College-commissioned oral history project.
McGuinness, a former IRA commander who today is the senior Catholic in Northern Ireland’s unity government, told a street rally in Catholic west Belfast that police would fail to prove IRA membership claims against Adams, as last happened in 1978, when Adams was arrested in the wake of a hotel firebomb that burned 12 Protestants to death.
‘‘That case was based on hearsay, gossip, and newspaper articles. It failed then, and it will fail now,’’ McGuinness said in front of a newly painted mural of a smiling Adams beside the words, ‘‘Peacemaker Leader Visionary.’’ Supporters at the rally — which took place across the road from the site of McConville’s abduction — held signs showing Adams meeting Nelson Mandela in South Africa.
Aides to Adams and McGuinness said Catholic west Belfast residents with IRA affiliations had been approached by police asking them to make statements about their knowledge of Adams’ IRA activities.
And 200 miles to the south, in the Republic of Ireland, an IRA veteran who served 31 years in prison for killing a policeman said a Northern Ireland detective knocked on his door seeking a witness statement. Peter Rogers, 69, said he refused.
Last month Rogers told the BBC he met both Adams and McGuinness in Dublin in 1980 to discuss their plans to smuggle stolen mining explosives from the Irish Republic to England for use in the IRA’s bombing campaign on London. Rogers said Adams was annoyed because he had failed to deliver them by ferry across the Irish Sea.
Rogers said he told Adams and McGuinness that the explosives were unstable and could detonate while being transported. He said Adams rejected his concerns.
‘‘Gerry said: ‘Look Peter, we can’t replace that explosive. You will have to go with what you have, and as soon as you can get it across, the better’ . . . I was given a direct order,’’ Rogers told the BBC.
Rogers, who in 1972 had escaped from a Royal Navy ship in Belfast used as a temporary prison to hold IRA suspects, was trying to move the explosives in October 1980 when two policemen stopped his van. He fatally shot one of the officers.
Before his Wednesday arrest, Adams rejected Rogers’s statements as false. Adams has always maintained he was never an IRA member.
According to every credible history of the modern Sinn Fein-IRA movement, he joined the outlawed group in 1966 and rose quickly through the ranks, becoming its Belfast commander in 1972, when he took part in the IRA’s first face-to-face truce negotiations with British government leaders in London.
British authorities have struggled to convict senior IRA figures of membership in an outlawed organization, a charge that carries a maximum five-year sentence. To be successful, prosecutions of IRA members usually require forensic evidence or witness testimony.
Commanders plan attacks but do not handle weaponry themselves to avoid creating forensic links. And people willing to testify against the IRA are rare.
Northern Ireland police successfully sued in US courts to get the audiotaped accounts from the Boston College project for use against Adams and others allegedly involved in the McConville slaying, which the IRA long denied but finally claimed in 1999. Her remains were found in 2003.
Reflecting Sinn Fein anger at police use of the Boston tapes, graffiti appeared Saturday in Catholic west Belfast denouncing those IRA veterans who talked as ‘‘touts’’ and ‘‘informers’’ — accusations that typically mean an IRA death sentence.