A Miami-Dade judge declared a mistrial Wednesday in the murder trial of former social worker's aide Grady Nelson, accused of fatally stabbing his wife and raping and stabbing his stepchildren.

The decision came after Miami-Dade police Detective Danny Aiken testified that Nelson told him during an interview that he had once been jailed for violating probation on a sexual-battery charge.

Circuit Judge Yvonne Colodny ruled that the testimony suggested Nelson had an earlier conviction -- which jurors were not supposed to know. Nelson had been convicted in 1991 of raping a 7-year-old neighbor.

Nelson, 52, is accused of killing his wife, Angelina Martinez, 32, in January 2005. He confessed to police. The trial began in early November.

-- DAVID OVALLE