Sinn Fein's deputy leader, Martin McGuinness, said he had been told by Adams' 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."
Other aides to Adams and McGuinness said Catholic west Belfast residents with IRA affiliations had been approached by police recently, asking them to make statements about their knowledge of Adams' IRA activities.
And 200 miles (320 kilometers) to the south, in the Republic of Ireland, an IRA veteran who served 31 years in prison for murdering 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.
He said he told both Adams and McGuinness that the explosives were unstable and could explode 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' claims 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, given the likelihood they will face death threats and be forced to flee their homes.
Sinn Fein said most of the detectives' questions being posed to Adams concerned allegations made by IRA veterans in a Boston College-commissioned oral history project. Northern Ireland police successfully sued in U.S. courts to get those audiotaped accounts 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 grass-roots Sinn Fein anger at police use of the Boston College tapes, graffiti appeared Saturday in Catholic west Belfast denouncing those IRA members who talked on tape as "touts" and "informers" — accusations that typically mean an IRA death sentence.
McGuinness has warned that Sinn Fein could reconsider its support for Northern Ireland's police force — a key 2007 commitment that enabled Sinn Fein to form a power-sharing government in Northern Ireland with Protestant leaders — if Adams is charged with a crime.
Should Sinn Fein withdraw support for the police, it would likely end Protestant cooperation with Sinn Fein and lead to early elections for the Northern Ireland Assembly. It also could fuel support for more IRA attacks on police, who in recent years have been able to operate with much less risk of attack in the IRA's working-class Irish Catholic power bases.
In a fresh reminder Saturday that some IRA members remain armed, a Belfast man was arraigned on charges of possessing a cache of Semtex, a powerful Czech-made plastic explosive that the IRA was supposed to have surrendered to authorities nearly a decade ago.
Thomas Hughes, 47, was ordered held without bail after police found about 2.5 kilograms (5.5 pounds) of Semtex in his Belfast apartment, the biggest such find of the explosive in the capital in a decade. Police said it was enough to make about 20 under-car booby trap bombs, which are most commonly placed under the driver's seats of police officers' private cars.
The major IRA faction, the Provisionals, killed nearly 1,800 people — including 284 police officers — before renouncing violence and disarming in 2005. But some of their Libyan-supplied weaponry, most notably Semtex, was seized by breakaway factions that oppose Sinn Fein's role in governing Northern Ireland.