WASHINGTON (AFP) - A US court dropped murder charges Thursday against a German-born woman who spent 23 years on death row over allegedly orchestrating the murder of her four-year-old son, news reports said.

Debra Milke, now 49, was charged and initially convicted of arranging the murder of her son Christopher in 1989 to collect insurance money.

Her conviction and death sentence in Arizona were thrown out in 2013 by a federal appeals court.

Prosecutors and police believe that in December 1989, Milke recruited her roommate, a would-be suitor and his friend to kill her 4-year-old son to collect an insurance policy on the boy.

The federal appeals court said the original prosecutor failed to reveal evidence that could have helped her challenge a detective who said she had confessed to him, the Arizona Republic newspaper reported.

Milke had denied confessing, and no confession was recorded.

But defence attorneys in 1990 could not obtain the detective's personnel record, which showed he had engaged in misconduct in other cases.

The federal appeals court said she should be released or retried and Maricopa County attorney Bill Montgomery opted to take her back to trial.

Lawyers for the woman said the prosecutor's conduct was egregious and the case should be thrown out altogether as trying her again would amount to double jeopardy, they argued.

The Arizona Court of Appeals agreed with this on Thursday, although it stopped short of saying Milke was innocent.

"Our analysis is based entirely on whether double jeopardy applies to bar Milke's retrial in this case, and we express no opinion regarding her actual guilt or innocent," the judges wrote.

The two men who have been convicted of actually killing the boy are currently on death row, according to the newspaper.