Dozens of officers — many sporting full riot gear with flame-retardant boiler suits, body armor, helmets and shields — confronted the hardline Protestants, many covering their faces, as they tried to block Adams' exit by sitting down in the roadway. After a 15-minute standoff, police escorted Adams out via a rear exit that the protesters could not see.
Sinn Fein said Adams intended to hold a press conference in a Belfast hotel within hours.
The investigation of Adams is not over, however, because police said they have sent an evidence file to Northern Ireland prosecutors for potential charges later.
Sinn Fein said detectives questioned Adams about audiotaped interviews that IRA veterans gave to a Boston College oral history project. Some interviewees accused him of being the Belfast IRA commander who ordered the abduction, killing and secret burial of Jean McConville in 1972.
Police faced a Sunday deadline to charge or release Adams or seek a judge's permission to extend his detention, a step they had already taken once on Friday when an initial deadline was due to expire.
The IRA did not admit responsibility for killing McConville until 1999, when the underground organization defended its action by claiming she had been a British Army spy. Her remains were found accidentally in 2003 near a Republic of Ireland beach. An investigation three years later by Northern Ireland's police complaints watchdog found no evidence she had been a spy.
Sunday's outcome — freedom but no official exoneration, with evidence bound for the Public Prosecution Service — suggested police do believe Adams was an IRA commander, but do not have strong enough evidence to charge him with this. Police last charged Adams with IRA membership in 1978 following a firebomb attack on a hotel near Belfast that killed 12 Protestants, but those charges were dropped.
British state prosecutors in Belfast would provide a second opinion. They could tell police either that no case could succeed based on existing evidence, recommend new avenues of investigation to strengthen the chances of a successful prosecution, or determine that charges should be filed. Typically however, when such evidence files are sent by police to prosecutors for complex terror-related cases, charges do not follow.
Adams has always denied membership of the outlawed IRA. His arrest weeks ahead of elections in both parts of Ireland infuriated his Irish nationalist party, which represents most of the Irish Catholic minority in Northern Ireland and is a growing left-wing opposition force in the Irish Republic.
Sinn Fein warned it could withdraw its support for law and order in Northern Ireland, a threat condemned Sunday by the Protestant leader of the province's power-sharing government, First Minister Peter Robinson.
Speaking Sunday before Adams' release, Robinson accused Sinn Fein of mounting "a despicable, thuggish attempt to blackmail" the police.
"I warn Sinn Fein that they have crossed the line and should immediately cease this destructive behavior," Robinson said, suggesting that the future of Northern Ireland's government was at stake.
Robinson's Democratic Unionist Party agreed to share power with Sinn Fein in 2007 on condition that the IRA-linked party accepted police authority. A former IRA commander, Martin McGuinness, serves as deputy leader. Such cross-community cooperation following four decades of bloodshed was the central goal of the U.S.-brokered Good Friday peace accord of 1998.
Robinson accused Sinn Fein of hypocrisy by demanding criminal investigations of killings committed by Protestant militants, the police and British Army, but rejecting any such investigations into the IRA, which killed nearly 1,800 people during a failed 1970-1997 campaign to force Northern Ireland out of the United Kingdom.
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