Sarah Palin has accused presidential candidate Barack Obama of "palling around" with terrorists - referring to his acquaintance with a former member of the Weather Underground. So who were the Weather Underground?
Bill Ayers, a "Weatherman" in 1970s, is now an education lecturer
In the late 1960s, the US government - embroiled in an unpopular war in Vietnam and with many of the grievances of the civil rights movement still unanswered - was facing widespread protests.
Often those who rebelled were rich in idealism and the action they took was largely peaceful.
However, on 8 October 1969 a newly-formed group of left-wing extremists, dubbed the Weathermen, went on the rampage in a well-planned protest in Chicago - the so-called Days of Rage riots.
A police station in the city was bombed and protesters engaged police in combat on the streets. More than 250 of the rioters were arrested, and the FBI began to follow the movements of the Weathermen very closely.
They were a splinter group from Students for a Democratic Society (SDS) - a product of the student radicalism endemic on college campuses in the late 1960s.
The group had recently published an article in the society's newspaper rejecting peaceful protest in favour of communist revolution. The article signed off with the Bob Dylan lyric: "You don't need a weatherman to know which way the wind blows."
According to the FBI, several of their members had travelled to Cuba and North Vietnam during 1969.
'Freaks are revolutionaries'
After the Chicago riots, with many of their members in jail, the Weathermen regrouped and rethought their tactics.
The group's communiques still betrayed obvious links to the counter-culture of the 1960s.
In May 1970 they issued a statement entitled A Declaration of a State of War, which declared: "Freaks are revolutionaries and revolutionaries are freaks.
Mr Ayers' wife, Bernardine Dohrn, is also now a lecturer
"If you want to find us, this is where we are. In every tribe, commune, dormitory, farmhouse, barracks and townhouse where kids are making love, smoking dope and loading guns—fugitives from Amerikan [sic] justice are free to go."
Later that year, they helped LSD advocate Dr Timothy Leary to break out of a Californian jail and flee the country.
But their goals and tactics were far out of kilter with most of those involved in the anti-war movement.
By the end of 1969 they decided to go underground and resort to bombing strategic targets - later changing their name to the Weather Underground Organization.
From 1970 to 1975 the group bombed police stations and patrol vehicles and court and government buildings.
In 1970 there were fatalities - a police officer died from his injuries after a pipe bomb was detonated in a San Francisco police station, while three of the group blew themselves up while building explosives in their New York apartment.
In recent articles and books, the group's leaders have claimed that after these incidents, they meticulously avoided putting people's lives at risk.
This claim is hotly disputed - that their bomb attacks claimed no more lives is often put down to luck more than judgement.
The group's most audacious attacks came in 1971, when they bombed the US Capitol, and a year later when they attacked the Pentagon.
The group splintered in the mid-1970s and ceased to be regarded as a threat by the authorities.
Many of its members became prominent professionals in US public life.
Bernardine Dohrn, the author of the Declaration of War, is now a law lecturer. Her husband, Bill Ayers, lectures in education.
Both were implicated in the group's most serious attacks, but were never convicted.
During the late 1990s, Mr Obama served on the same charity board as Mr Ayers.
Such was the threat engendered by the group that a tenuous association with a former member can still cause ripples in a presidential race three decades later.