DENVER (AP) — Lawyers for Oklahoma City bombing conspirator Terry Nichols were headed back to federal court Wednesday to ask a judge to throw out their client's conviction and life sentence.

Nichols, 45, was found guilty of conspiracy and eight counts of involuntary manslaughter in the 1995 federal building bombing, which killed 168 people. His lawyers are asking U.S. District Judge Richard Matsch to vacate the conviction and sentence.

Convicted bomber Timothy McVeigh, who was sentenced to death, dropped all appeals in December and is scheduled for execution May 16. About 250 people who were injured or lost relatives in the bombing have asked to witness his execution.

Nichols' most recent bid for a new trial was rejected in December when a federal appeals court ruled the FBI did not withhold crucial evidence.

This time, his lawyers are arguing the jury wasn't properly instructed on the charge of conspiracy to use a weapon of mass destruction. In court documents, defense lawyer John M. Richilano argued the conspiracy law is unconstitutional because it allows a judge, instead of a jury, to assess facts that could result in the death penalty.

Prosecutors said the argument is irrelevant because Nichols received a life sentence, which the conspiracy statute allows independently of considerations of the death sentence.