A former Rikers Island guard admitted to killing his girlfriend's mother.

Ralph Keppler, of Lynbrook, pleaded guilty to second-degree murder.

Prosecutors say Keppler used a barbell to repeatedly strike Theresa Kiel in the head outside the victim's home in 2016.

They say the attack was allegedly over a business dispute.

Kiel was in a vegetative state for two years before she died.

Kiel’s daughter, who was dating Keppler, has also been charged.