DeKalb County police have charged two men — who were already in jail on unrelated cases — with murder and aggravated assault in the killing of a 9-month-old boy last month, the most recent shooting in a string of violence that started with a May 3 house party.

DeKalb County Public Safety Director Cedric Alexander said Marco Watson, 36, and Devin Thomas, 18, were each charged with one count of murder and three counts of aggravated assault for killing Kendarious Edwards and critically wounding the baby’s mother, grandmother and a family friend during a May 10 home invasion.

“At this point, we know these two were there,” Alexander said at a Thursday morning news conference.

He declined to say if Watson or Thomas pulled the trigger.

Alexander also said detectives don’t know if anyone else was involved and the investigation remains open.

Alexander said the $7,000 reward did not lead to the charges against Watson and Thomas. The investigation led detectives to Watson and Thomas. Watson was in jail for a probation violation and Thomas was in jail on a charge of making a false statement.

The first fatal shooting was on May 3, when Michael Phillips, 29, was killed.

A week later, Alexis Malone, a possible witness in Phillips’ shooting, was found dead. Thomas may have been a friend of Malone’s, Alexander said.

Two people have been charged with her killing and police have said the shooting of the 9-month-old may have been in retaliation for Malone’s death.

Alexander has called the brutality and brazenness of the spree insane. He said today the investigation is active and ongoing and would not provide many specific details regarding the arrest.

“Our goal is to keep this (county) safe. If a body drops … we’re going to find out who you are,” Alexander said.

“We’re not going to tolerate this western-style mentality” of seeking revenge for justice, he said.