Police in Mobile have arrested a third person in the killing of Cailee Knight, a 9-year-old girl who was shot while sleeping inside her home on Tuesday.

Darius Janoski Lucky Jr., 18, is held in the Mobile Metro Jail on a felony murder charge, records show.

“This is an ongoing investigation,” said Mobile police Cpl. Katrina H. Frazier in a statement announcing the arrest on Saturday.

The authorities said Knight was asleep on a couch when gunshots came into the home. Though several other children were inside, no one else was reported injured.

Keith Blackwood, the district attorney of Mobile County, has described the shooting as “horrifying and heartbreaking.” He said on Friday that he plans to seek enhanced penalties for Knight’s killers.

Earlier this week, police arrested two other suspects: Tyrone Deairus Williams, 19, and Arieal Rapheal Curry, 22. Williams and Curry are charged with felony murder and shooting into an occupied building.