One of the Baltimore police officers who arrested Freddie Gray has challenged police and a top prosecutor to produce a knife that prompted the arrest, arguing in a court motion that it is an illegal weapon.

Attorneys filed the motion in Baltimore District Court for Officer Edward Nero, who is charged with second-degree assault, misconduct in office and false imprisonment. The motion appeared to challenge the basis for charges Nero faces after the arrest of Gray, a black man who died a week after suffering a severe spinal injury in police custody.

Last Friday, Baltimore City State's Attorney Marilyn Mosby charged Nero and the others just a day after receiving the investigative report from the police department. Mosby said the officers had unlawfully arrested Gray because the knife he had in his pocket is considered legal under Maryland state law.

Marc Zayon, Nero's attorney, argued in the motion filed Monday that the knife in Gray's pocket — described in charging documents as "a spring assisted, one hand operated knife" — is in fact illegal under state law. Maryland, he said, defines a knife as unlawful if it opens automatically by pushing a button, spring or other device in the handle.

The charges against the officers came near the close of a turbulent week in which violence, looting and fires erupted in the streets April 27 only hours after Gray's funeral that Monday.

But some lawyers including Andy Alperstein, who has represented police officers but is not involved in the Gray case, said those charges can only be proven if Gray was wrongly arrested. If the knife was illegal, "there is no case" against Nero and another officer, he said.

"If the facts were that the knife was illegal then the Gray arrest would be justified. Even if it wasn't illegal and the officers acted in good faith, it would be the same result. All charges fail," Alperstein said.

Some spring-assisted knives are opened by pushing a thumb stud attached to the blade.

Many knives have these spring-assisted opening mechanisms but are not the automatic knives prohibited under Maryland law, said Michael Faith, marketing director for Henderson's Sporting Goods in Hagerstown.

"An automatic knife means all you do is push a button and the blade pops out," Faith said. "A lot of knives will have a little spring assist so when you push it open with your thumb, the knife will open up pretty much by itself."

Police said officers chased Gray two blocks after making eye contact with him and subsequently found the knife in his pocket.

The Associated Press has made repeated requests to the police department for a physical description of the knife as well as photographs. Police later referred the request to the state's attorney's office.

Nero and Officer Garrett Miller are charged with misdemeanors. Four others — Sgt. Alicia White, Lt. Brian Rice and officers Caesar Goodson and William Porter — are charged with felonies ranging from manslaughter to second-degree "depraved-heart" murder.

Calls to attorneys representing Miller and Rice, who were involved in Gray's arrest, were not immediately returned.

Mosby's office declined to comment on a pending case, citing prosecutorial ethics.

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Associated Press writer Ben Nuckols contributed to this report.