An appeals court in the U.S. state of Wisconsin refused to overturn the conviction of a prisoner who said no one ever saw him write a threatening letter to a judge, ruling that he put his name and return address on the envelope.

The 2nd District Appeals Court issued its decision Wednesday in the case of Anthony Dwane Turner.

A jury convicted Turner in 2004 of sending a letter to Waukesha County Circuit Judge James Kieffer telling him he was going to kill him, according to computerized court records and a criminal complaint. Kieffer had sentenced Turner to 15 years in prison a year earlier on assault-by-a-prisoner and reckless injury charges.

The judge told investigators he believed Turner was a serious threat, according to the complaint. Turner got six more years tacked onto his sentence.

Turner asked the appeals court in 2005 to overturn his conviction. He argued prosecutors did not present any evidence at trial that anyone saw him write the letter or that the handwriting was his.

The appeals court refused, saying the envelope was marked with Turner's name, inmate number and the address of the prison in Green Bay.