A photograph of actor John Travolta signing an autograph for Drew Peterson, the prime suspect in fourth wife Stacy's disappearance, could end up being evidence in the weapons case brought against him by police.

Peterson was arrested last week on a gun violation charge for owning and using an assault rifle with a barrel shorter than required by state law.

The 54-year-old former Bolingbrook, Ill., police officer has contended that he used the gun on the job, and the law applies only to civilians.

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On Thursday, his attorney released a photo of Peterson carrying the weapon in question as Travolta signed an autograph for him. Peterson had been assigned to protect the star while he was shooting the movie "Battlefield Earth."

"This photo should convince the state police that they've made a mistake with these charges," Joel A. Brodsky, Peterson's criminal defense attorney, said in a press release he circulated with the photo on Thursday.

Brodsky said the picture proves that his client did use the assault rifle while he was on duty as a sergeant.

He joked that if the photo doesn't persuade police on its own, "we might need to subpoena Mr. Travolta when this case goes to court."

A call to Travolta's agent was not immediately returned.

Illinois State Police have charged Peterson with a class 3 felony gun charge. He turned himself in on May 21, was booked and then was released on bail.

Last week, a judge ordered authorities to return eight of the 11 guns they confiscated from Peterson during their investigation. The judge said Peterson had the right to transfer ownership of the weapons to his son.

Peterson faces up to five years in jail if he is convicted of unlawful use of a weapon. His next court date is May 30.

Stacy Peterson, 23, was reported missing on Oct. 29, 2007, after she failed to show up at a friend's house and her family couldn't reach her. Drew Peterson said he last spoke to her the previous night. He has maintained that he had nothing to do with her disappearance, saying she ran off with another man and is in hiding.

Authorities have said they believe Stacy Peterson was the victim of a homicide, and her husband remains the sole suspect.

Police are also investigating the death of Peterson's third wife, 40-year-old Kathleen Savio. She was found dead in a dry bathtub in 2004. Her death was initially ruled an accident, but when Stacy Peterson disappeared, Savio's body was exhumed. An additional autopsy determined she was the victim of a homicide.

Documents show that Savio had filed for an emergency order of protection from Peterson, writing in the file that "he wants me dead and if he has to he will burn the house down just to shut me up."

Peterson retired from the police force after his fourth wife vanished.

FOX News' Catherine Donaldson-Evans and The Associated Press contributed to this report.