(TMZ) -- Elle King's husband was arrested after they had a violent altercation about a month ago -- when TMZ had no idea they were married -- but she chose to let him off the hook, criminally.

Law enforcement sources tell TMZ the incident went down April 23 at Elle and Andrew Ferguson's L.A. home. TMZ was told they were arguing when Ferguson grabbed her by the throat with both hands, tossed her on a bed ... and said, "I'm gonna f***ing kill you."

TMZ was told cops found scratches on Elle's neck and marks on her arm. Ferguson was arrested and booked for felony domestic violence. TMZ was told his side of the story was that he threw Elle to stop her from hitting her head on a wall.

Elle later told cops things just got out of hand while they were partying, and she did not wish to press charges. The D.A. ultimately declined to move forward with the case ... however, the arrest adds context to the singer's surprise breakup announcement.

Elle posted a pic Monday, revealing they'd secretly married back in March 2016, and were going to have a big ceremony on April 15, 2017. Elle admitted skipping that to go to a Eagles of Death Metal concert instead. It's unclear if the fight, 8 days later, was related.

TMZ has reached out to Elle, but no word back.