Mr. Martin has denied involvement in the three killings. His lawyer, Michael Thompson, said his client would plead not guilty at an arraignment next week and questioned why an indictment would come so long after the crime. “That’s what we’re all curious about,” he said. “What evidence do they have that they didn’t have almost four years ago?”

The arrest on Saturday was unusual in part because of its setting. Learning that a pilot had been arrested in a murder case put some passengers in the Louisville airport on edge. “I felt like it was a little chaotic and unorganized,” Ashley Martin, who had been waiting to board a flight at the time of the arrest, told WDRB, a local television station. “You could feel that energy. You could feel the tension from flight attendants.”

Mr. Martin has made headlines before, and his case has a complicated history.

In 2015, Mr. Martin, a combat veteran, was fighting his dismissal from the United States Army in a military court case that became well known among certain circles, including men’s rights groups. Mr. Phillips had been expected to serve as a witness in that case — until he was found dead.

The military case began when a woman told the authorities that Mr. Martin had mishandled classified information, according to past and present lawyers for Mr. Martin. He has said that he thought the woman was his wife until he learned she was already married to someone else, and that she had also had an affair with Mr. Phillips.

The accusations against Mr. Martin eventually led to his dismissal from the Army, where he had served as an aviation pilot with a rank of major. (A dismissal is like a dishonorable discharge, but for officers.) He fought that dismissal, even as the case grew to include charges that he had sexually assaulted a child.