BALTIMORE — 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.