Mr. Killen, 80, who had been free on bond, was immediately taken into custody. He faces up to 20 years on each of the three manslaughter counts. Sentencing was set for 10 a.m. on June 23.
Relatives of the victims said at a televised news conference that the trial was an important step but that the lesser conviction demonstrated the need for justice for the victims of crimes committed during the civil rights era.
"The fact that some of these jurors have lived all these years, and could not bring themselves to recognize that these were murders, indicates that there are still some people among you who choose to look aside, who choose to not see the truth," said Rita Bender, the widow of one of the victims, Michael Schwerner.
Ben Chaney, the younger brother of victim James Earl Chaney, said that the trial had shed light on social and economic problems related to race that persist today.
"The light will shine on this state, on this community - lightly - but we are still living in the dark," he said. "There's a lot of work to be done. This is not over with."
The prosecutor, Mark Duncan, the district attorney of Neshoba County, said that he still believed that murder would have been the right verdict, but that he understood the jurors' misgivings.
"I think it was asking a lot of a jury to convict a man based on the testimony of people they can't see, who are on paper, so I can't criticize the jury at all," Mr. Duncan said. "I understand the position that they were in." However, he added, "I feel like he's guilty of murder, yes."
One after another in the last decade or so, a new generation of southern prosecutors, prompted by news reports, victims' families or even their own youthful memories, has reopened some of the most notorious cases from the civil rights era. In 1994, for example, Byron de la Beckwith was convicted in the 1963 assassination of the Mississippi civil rights leader Medgar Evers. More recently, the body of Emmett Till, a 14-year-old black boy who was kidnapped and slain in Mississippi in 1955, was exhumed by prosecutors revisiting that case.
With Mr. Killen's sentencing, another of what some have called these southern "atonement trials" - the legal system's revisiting of the most notorious atrocities of the civil rights era - will be brought to an end.
The jury, which began deliberating Monday afternoon, reported just before breaking for the night that they were spilt 6 to 6 on the case against Mr. Killen, an ailing, 80-year-old sawmill operator who was charged with masterminding the 1964 slayings. The jurors resumed their deliberations this morning, after spending the night sequestered at a hotel on the order of the judge, Marcus D. Gordon of State Circuit Court in Neshoba County.
Mr. Killen, the first to face state murder charges in the case, did not testify at his short trial, which began last Wednesday. Television coverage of the proceedings today showed Mr. Killen breathing with the aid of an oxygen tube for the first time during the trial and looking straight ahead as he listened to the reading of the verdict and the judge's confirmatory poll of the jury of nine whites and three blacks.
On the night they disappeared, the three victims, all in their 20's, had been helping to register black voters during the "Freedom Summer" of 1964 and were investigating a church in Philadelphia, Miss., that had been burned by the Ku Klux Klan. The victims, Mr. Chaney, Mr. Schwerner and Andrew Goodman, were initially taken into custody on speeding charges. Upon their release from jail, their car was pursued by Klansmen. They were shot dead and later found buried in an earthen dam.
Their 44-day disappearance thrust the Jim Crow code of segregation in the South into the national spotlight and helped to spearhead passage of the Civil Rights Act of 1964.
Of the 18 men tried a year later on federal civil rights charges, 7 were convicted by the all-white jury. Mr. Killen was freed when the jury deadlocked 11 to 1 in favor of conviction, after a holdout juror said she could not convict a preacher.
Eight of the defendants are still alive. The men who were convicted were sentenced to prison terms ranging from 3 years to 10 years, although none served more than 6 years. The case gained renewed international attention when it was dramatized in the 1988 movie "Mississippi Burning."
Mr. Killen, who was tried by the State of Mississippi on three counts of murder, was free on bail and was using a wheelchair because of arthritis that worsened after he broke his legs in a tree-cutting accident in March.
Prosecutors sought to convince the jurors that although Mr. Killen was not present during the killings, he had organized groups of men and planned what they would do, including, according to testimony, telling them where to go to bury the men and instructing someone to buy rubber gloves to wear during the crime.
Mr. Duncan, the prosecutor, said during his closing arguments that the evidence that Mr. Killen was culpable in the killings is "absolutely overwhelming."
"There is only one question left," Mr. Duncan said. "Is a Neshoba County jury going to tell the rest of the world that we're not going to let Edgar Ray Killen get away with murder? Not one day more."
