YOKOSUKA NAVAL BASE, Japan — A Navy officer accused of attempted sexual assault of a minor following an Okinawa sting operation was acquitted of five related charges by a court martial jury on Thursday.

Lt. Stephen M. Kimball was freed after spending just over six months in pretrial confinement, Navy officials said.

The acquittal came after military judge Col. Eugene Robinson ruled video of an interview conducted by the Naval Criminal Investigative Service just after Kimball’s arrest was inadmissible, due to 80 minutes of missing footage that arose only after the trial began.

“The recording of the interrogation taken by NCIS was edited by the government through its own deliberate actions,” Robinson’s instructions to the jury stated. “An additional portion was not provided to the defense until after the testimony of (an undercover agent). A portion is still unaccounted for.

“Thus, the government failed to comply with the Military Rules of Evidence and the Rules for Courts-Martial.”

The defense also argued that portions of text messages between the undercover agent and Kimball were lost or destroyed. Jurors were allowed, if they chose, to “infer that the contents of those text messages were unfavorable to the government,” according to the instructions.

The missing video was an unintentional transcription mistake made by an investigator, Commander Navy Region Japan spokesman Cmdr. Ron Flanders said Friday. The prosecution made additional footage available to the defense once it was discovered, he said.

“There was no intent to suppress anything,” Flanders said. “There was nothing on there that was damning for the prosecution that needed to be suppressed.”

Robinson denied a defense motion for a mistrial based on the missing footage.

Timothy Bilecki, Kimball’s defense attorney, said the content ultimately revealed in the video wasn’t the problem.

Bilecki had made the question of the missing video a key part of his client’s defense prior to it being found. Introducing the video mid-trial violated discovery rules, Bilecki said.

Kimball had been on temporary duty at Kadena Air Base, Okinawa, when he met an undercover Navy agent online, who claimed to be 15, Bilecki said.

The two exchanged text messages for about 20 days, Bilecki said. The agent sent at least four videos and 18 photos to Kimball. However, Bilecki said that several photos showed the agent as a 22-year-old, which could have sent his client a mixed message.

Kimball arranged to meet the agent at the Olympic Mall on base and was arrested upon arrival.

The arrest was one of dozens in a multiyear NCIS operation aimed at catching personnel interested in sex with minors on Okinawa, where about half of all U.S. forces in Japan are based. Bilecki’s Hawaii-based firm has represented multiple defendants in such cases.

“I don’t have an issue if law enforcement wants to run an undercover operation where they are targeting servicemembers seeking underage sex online,” Bilecki, a former military attorney, said Friday. “The issue our law firm has is when they use … forms of inducement and entrapment to arrest otherwise innocent individuals who are not looking to meet minors.”

The Navy in Japan referred questions Thursday about the sting operation to NCIS in Washington. NCIS had not responded to questions by Friday.