The PPS said Adams and six other people will not be prosecuted in the abduction and death of Jean McConville, a Belfast resident and widowed mother of 10 who was reportedly killed by the IRA because the group believed she was a spy for the British army.
Prosecutors concluded that evidence relating to seven people in the case "is insufficient to provide a reasonable prospect of obtaining a conviction against any of them for a criminal offense," said Pamela Atchison, the PPS deputy director.
Adams has long denied having any role in the death of McConville, who authorities said was taken from her Belfast home in 1972 when she was 37. Her remains were found partially buried on a beach in Ireland's County Louth in 2003. She died of a single gunshot wound to the back of the head.
An eighth person questioned in the case was charged in March 2014 with soliciting McConville's murder. That prosecution will continue, Atchison said.
Boston College interviews led to 2014 questioning
Adams is a prominent Catholic politician who helped broker peace in Northern Ireland and who has long been associated with the IRA, once considered the armed wing of Sinn Fein. Today, Sinn Fein is an opposition party in the Republic of Ireland and is the second-largest party in Northern Ireland's legislature.
The IRA admitted in 1999 to killing a number of people who became known as "the disappeared," those who vanished during a 30-year conflict known as the Troubles. That conflict was between Protestant loyalists who wanted Northern Ireland to remain part of the United Kingdom and Catholic nationalists who wanted to see the north united with Ireland.
Northern Ireland police brought Adams in for four days of questioning last year after the release of interviews given by members of the IRA, who allegedly implicated Adams.
The recordings of those interviews were made by Boston College as part of the Belfast Project, which is a collection of interviews conducted with former Northern Irish paramilitary fighters, who provided an oral history of decades of fighting.
Participants in the project were told their recorded interviews would be kept secret until their deaths. Last year a U.S. court ruled that tapes of deceased interviewees that contained claims about the McConville killing be given to police in Northern Ireland.
"Let me be very clear: I am innocent of any involvement in any conspiracy to abduct, kill or bury Mrs. McConville," Adams told reporters after police questioned him in May 2014.
Before that, Adams released a statement asserting that McConville's death and secret burial "was wrong and a grievous injustice."