The news came after less than three hours of deliberations.
The forewoman in the trial of Edgar Ray Killen announced the 6-6 split to a stunned courtroom Monday night, Court TV reported. But she did not refer to the impasse as a "deadlock," and jurors are due back in deliberations Tuesday morning.
The announcement came after the judge tried to determine whether to recess the jury for the evening, and called the group into court to ask for a tally of their most recent vote.
The forewoman did not say whether the panel was split over whether to convict or acquit Killen, or whether to convict him of murder or manslaughter.
Civil rights workers James Chaney, Andrew Goodman and Michael Schwerner were beaten and killed by a white mob on June 21, 1964.