The actor Alec Baldwin fiercely insisted he was not to blame in the fatal shooting of a cinematographer on the set of a Western being filmed in New Mexico, claiming that another person had accidentally placed a live round in the gun that went off in his grasp as he was rehearsing a scene.

“Someone put a live bullet in a gun, a bullet that wasn’t even supposed to be on the property,” Mr. Baldwin said in a television interview that was broadcast on Thursday night. “Someone is ​responsible for what happened, and I can’t say who that is, but I know it’s not me.”

Mr. Baldwin made the comments in an emotional ABC News interview with George Stephanopoulos, the first time that Mr. Baldwin has publicly given an account of what happened in October. The actor’s description of the episode may cast greater scrutiny on crew members and suppliers and the question of who was responsible for safeguarding firearms in the low-budget production.

In the interview, excerpts from which had been released on Wednesday, Mr. Baldwin also said that he did not pull the trigger of the gun he was practicing with on the set of “Rust” when it fired a live round.

“I would never point a gun at anyone and pull a trigger at them — never,” Mr. Baldwin said.

The fatal shooting took place on Oct. 21 near Santa Fe, N.M., on a movie set designed to be a church. Mr. Baldwin was practicing drawing an old-fashioned revolver that he had been told contained no live rounds when it suddenly fired, killing the film’s cinematographer, Halyna Hutchins, 42, and wounding its director, Joel Souza, 48.

Mr. Baldwin said that he was stunned by what happened and that at least 45 minutes passed after the gun went off before he realized that it could have contained a live round.

“I stood over her for 60 seconds as she just laid there kind of in shock,” Mr. Baldwin said.

The actor added that he did not cock the hammer of the gun, but pulled it back as far as he could and let it go in an action that might have set it off. “I let go of the hammer — bang, the gun goes off.”

Investigators are seeking to determine how a live round got into the gun that Mr. Baldwin was practicing with, why the crew members who inspected it on set failed to notice, and why the gun fired.

Mr. Baldwin’s contention that he had not pulled the trigger was supported by a lawyer for the film’s assistant director, Dave Halls, who had been standing near Mr. Baldwin inside the church set when the gun fired.

The lawyer, Lisa Torraco, told the ABC News show “Good Morning America” on Thursday that Mr. Halls had told her that “the entire time Baldwin had his finger outside the trigger guard, parallel to the barrel.” She said Mr. Halls had told her that “since Day 1, he thought it was a misfire.”

In the ABC interview, Mr. Baldwin also said he recalled that shortly before the shooting, Mr. Halls had told him, “This is a cold gun,” an industry term implying that firearm does not have live rounds and is safe to use.

“When he’s saying, ‘This is a cold gun,’ what he’s saying to everybody on the set is, ‘You can relax,’” Mr. Baldwin said.

Mr. Baldwin, who has come under intense criticism after the shooting, has already been questioned by detectives and is cooperating with the investigation. No one has been charged in connection with the shooting, and authorities have not placed blame on any individual.

“I got countless people online saying, ‘You idiot, you never point a gun at someone,’” Mr. Baldwin said. “Well, unless you’re told it’s empty and it’s the director of photography who’s instructing you on the angle for a shot we’re going to do.”

Some gun experts said it was possible that the gun, a single-action revolver, could have discharged without Mr. Baldwin’s pulling the trigger if he had pulled back the revolver’s hammer and released it before it was fully cocked. But they questioned whether that would have created enough force to fire the live round.

Clay Van Sickle, a movie industry armorer who did not work on “Rust,” said guns generally go off only when someone pulls the trigger. “Unless that gun was in a horrible state of disrepair,” he said, “there is no other way that gun could have gone off.”

As detectives work to trace the source of the live round, one focus has been on Seth Kenney, who supplied blanks and dummy rounds for the production.

According to court documents filed on Tuesday, detectives are trying to determine whether Mr. Kenney sent live ammunition as well as blanks and dummies, and they have searched his business in Albuquerque, PDQ Arm & Prop.

Mr. Kenney said in an interview that he was confident he was not the source of any live round.

“It is not a possibility that they came from PDQ or from myself personally,” he told “Good Morning America.”

Thell Reed, a weapons expert who has worked and consulted on a number of films, has told detectives that he supplied live rounds to Mr. Kenney for training on another film, according to court documents. Mr. Reed, who is the father of Hannah Gutierrez-Reed, the “Rust” armorer, suggested they might match live rounds found on “Rust.”

The film’s prop master, Sarah Zachry, has told investigators, according to the court documents, that the ammunition on the set had come from “various sources” — from Mr. Kenney, but also from Ms. Gutierrez-Reed, who was said to have brought some from a previous production, and from a person identified only as “Billy Ray.”

Since the fatal shooting, two crew members who were in the room when the gun went off have filed separate lawsuits, naming Mr. Baldwin, the film’s producers and other crew members including Mr. Halls and Ms. Gutierrez-Reed as defendants.

Both lawsuits say Mr. Baldwin should have checked the gun himself to see whether it was safe to handle. In the interview, Mr. Baldwin said that on the day of the shooting, one of the plaintiffs touched his shoulder and said he bore no responsibility for what happened.

Mr. Baldwin declined to say which plaintiff it was. Serge Svetnoy, one of the crew members who filed suit, told ABC that he did say that to Mr. Baldwin but later changed his mind.

The actor insisted that the tragedy occurred after he was handed the gun and was told it was safe, and that Ms. Hutchins herself had told him how to position it. Both he and Ms. Hutchins assumed the gun was safe to handle, he said.

“I am holding the gun where she told me to hold it,” Mr. Baldwin said. “I can’t imagine I’d ever do a movie that had a gun in it again.”

Matt Stevens contributed reporting.