Former Ku Klux Klan leader Edgar Ray Killen, 80, attended the first day of his trial in a wheelchair with broken legs suffered in an accident.
The former sawmill operator and part-time Baptist preacher is charged with being the ringleader in the deaths of Michael Schwerner, 24, Andy Goodman, 20, and James Chaney, 21.
Mr Killen was taken away on a stretcher suffering high blood pressure and breathing problems after testimony from Mr Schwerner’s widow Rita Bender.
The judge adjourned the trial in Philadelphia, Mississippi.
Ms Bender, 63, told the jury of the events that sent her husband from his New York home to Mississippi during the ‘Freedom Summer’ of 1964.
She said she knew the moment authorities found the vehicle the three had been travelling in, burned and abandoned in a swamp, they had been killed.
“I think it hit me for the first time that they were dead, that there was really no realistic possibility that they were alive,” she said, appearing close to tears at times.
The trial has gone into recess until at least Friday, pending Mr Killen’s ability to attend.
The three — two white New Yorkers and one black Mississippian — were in the town of Philadelphia investigating the burning of a black church, while taking part in efforts to help blacks enrol to vote.
They were stopped for speeding, jailed for a few hours, then released, before being ambushed by a group of Klansmen.
Their beaten and bullet-riddled bodies were found 44 days later in a dam.
The case drew worldwide attention following the 1988 Hollywood movie Mississippi Burning, starring Gene Hackman and Willem Dafoe.
Eighteen Klan members, including Mr Killen, were indicted in 1964.
Seven were convicted by an all-white jury and sentenced in 1967 to prison terms of three to 10 years.
But Mr Killen was acquitted, as one female juror reportedly said she could not convict a preacher.
The case was reopened in 2004 and Mr Killen could get life imprisonment if convicted.
However the men suspected of carrying out his orders to kill the three men are no longer alive.