DENVER -- The suspect accused of dragging his girlfriend behind his car has filed a motion saying he is mentally retarded and can't be tried for first-degree murder.

The attorney for Jose Luis Rubi-Nava filed the sealed document with the 18th Judicial District in Douglas County.

In a court hearing earlier this month, Rubi-Nava's attorney said he has an IQ of 65. That is lower than the standard generally used to classify someone as retarded.

Former prosecutor Karen Steinhauser told 7NEWS that IQ is not the only standard a judge uses when determining if someone is competent to stand trial.

"Just the fact the somebody has a low IQ doesn't mean that it automatically equates to the legal definition of competency," said Steinhauser.

Steinhauser said a judge must determine if a defendant understands what is happening to them and must be able to assist in their own defense.

The U.S. Supreme Court has ruled that a defendant must be competent to stand trial and face execution.

Rubi-Nava is accused of killing 49-year-old Luz Maria Francio-Fierros in September 2006.

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.

The case is one of the most gruesome that prosecutors have seen.

Rubi-Nava is accused of wrapping an orange nylon strap around his girlfriend's neck and dragging her behind his truck for at least a mile

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. Fierros was buried in Mexico.