NASHVILLE, Tenn. (AP) — A Tennessee inmate scheduled to be executed this month petitioned the courts Monday to reopen his case after DNA from an unknown person was detected on one of the murder weapons.

Oscar Smith, 71, is scheduled to die by lethal injection on April 21. He was convicted of fatally stabbing and shooting his estranged wife, Judith Smith, and her two teenage sons, Jason and Chad Burnett, 13 and 16, at their Nashville home on Oct. 1, 1989. He was sentenced to death by a Davidson County jury in July 1990 for the murders. Smith has maintained that he is innocent.

In a Monday filing, Smith’s attorney states that newly available touch DNA technology allowed the previously impossible analysis of evidence left on an awl — a leatherworking tool similar to an icepick — that was found at the crime scene. The victims were also shot and stabbed with a knife, although those weapons were never recovered. In January, the courts released the awl to Smith’s DNA experts upon agreement between Smith and the state. The analysis found DNA from an unknown person.

Tennessee law allows defendants to reopen their cases under certain circumstances, including where new scientific evidence establishes that they are actually innocent. The petition to reopen a case must be for “the purpose of demonstrating innocence and not to unreasonably delay the execution of sentence or administration of justice.”

Smith argues that although his execution date is near, he could not have presented the DNA evidence any sooner because the technology allowing it to be analyzed is brand new.

Smith previously sought to prove that fingerprint evidence used against him was unreliable. Crime scene investigators testified they found a bloody palm print on the sheet next to Judy Smith’s body that was missing two fingers — the same two fingers that Oscar Smith is missing on his left hand. A fingerprint expert hired by Smith later opined that the investigator made numerous errors and could not have definitively identified the print.

In one example, the investigator’s own fingerprint was found on the awl, demonstrating “incompetence and lack of professionalism,” according to Smith’s Monday filing. A panel of the Tennessee Criminal Court of Appeal last month upheld a lower court’s ruling rejecting Smith’s fingerprint analysis claims. Smith has appealed.

“While procedural technicalities have thus far prevented the courts from reviewing new evidence that shows the fingerprint examiner who identified Mr. Smith overstated the case, an unknown person’s DNA on the murder weapon must be fully considered by the courts,” Smith’s attorney Amy Harwell said in a statement Monday.

A spokesperson for the Tennessee attorney general’s office said in an email that the office had no comment on Smith’s motion to reopen his case.

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