WASHINGTON — A Navy doctor accused of secretly videotaping Naval Academy midshipmen engaged in sexual acts committed a “flagrant violation of trust,” military prosecutors said Monday as the doctor’s court martial began.

Cmdr. Kevin J. Ronan is accused of using a video camera hidden in an air purifier in his home to tape midshipmen he hosted at his house last year. He is charged with seven counts of conduct unbecoming an officer, three counts of illegal wiretapping and one count of obstruction of justice.

The Navy began its investigation in January after two men, one a midshipman the other a former student at the academy, turned over to authorities recordings they said they found in Ronan’s home.

In his opening statement, Navy prosecutor Lt. Justin Henderson said Ronan recorded midshipmen either with partners or alone in his spare bedrooms, edited them down to the sexually explicit content and transferred them to DVDs. He said thousands of gay pornographic images were found on Ronan’s home computer.

Ronan “violated the trust of his midshipman sponsorees,” Henderson told the jury of six Navy officers hearing the case at the Washington Navy Yard.

Ronan’s attorney, William Ferris, said Ronan did not make the videotapes and suggested that midshipmen, who had access to his client’s home computer, may have been responsible for the pornography found on Ronan’s hard drive.

Ferris said the tapes could have been part of an attempt by one of the two men who turned the tapes over to authorities to blackmail his client. He said the man flunked out in January and faced the prospect of having to repay the government thousands of dollars for his education. Ferris said the man asked his client for money and was refused.

Navy investigators found a purifier with a camera inside, a device typically used for monitoring nannies, in the attic of Ronan’s Annapolis home, Henderson said. He said the owner of the company that sold it said Ronan ordered an upgraded version with sound that could record in low light.

Ronan is assigned to the Navy’s Bureau of Medicine and Surgery but was previously a team physician for Naval Academy sports teams and a medical officer in the midshipman dorm. He also was designated as a midshipman sponsor by the academy, hosting roughly a dozen students under the program where midshipmen spend weekends off campus at private homes to relax.

The case is expected to last two weeks.