CASTLE ROCK, Colo. -- A man accused of wrapping a nylon strap around his girlfriend's neck and dragging her to death behind a car pleaded not guilty Friday, and his lawyer said he is mentally retarded.

Jose Luis Rubi-Nava, 36, is charged with first-degree murder and second-degree kidnapping in the death of Luz Maria Franco Fierros, 49.

Defense lawyer Tamara Brady said Rubi-Nava has "significant" mental retardation and has an IQ of 66. She planned to file a motion seeking to have him declared mentally retarded by the court. Kathleen Walsh, a spokeswoman for prosecutors, declined comment.

If convicted of first-degree murder, Rubi-Nava would face life in prison or the death penalty. He couldn't be executed if he is found to be retarded.

Franco Fierros' mangled body was found early Sept. 18 in a quiet subdivision off Interstate 25, north of Castle Rock, 20 miles south of Denver. An autopsy report said she died from strangulation and massive head wounds.

Investigators have said Rubi-Nava told them he had dragged her behind his car after a violent argument, when he accused her of seeing other men.

Both the suspect and the victim were from Mexico and were living together in Glendale, Colo. Authorities have said Rubi-Nava was in the country illegally and he also faces charges of possessing and using fraudulent government documents.

Fierros was buried in Mexico.