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On Mother's Day, Youlander Ross wept for her murdered son, rookie Hattiesburg, Mississippi, police officer Liquori Tate.

"They took away my baby," Ross said in tears Sunday, hours after Tate, 25, was gunned down while providing backup on a Saturday night traffic stop. The officer he was assisting, Benjamin Deen, 34, was also killed.

Despite the "unbearable" pain, Ross told NBC News she forgave her son's killers.

Four people have been arrested, police said: Marvin Banks, 29, charged with two counts of capital murder, grand theft auto and illegal weapons possession; Joanie Calloway, 22, charged with two counts of capital murder; Curtis Banks, Marvin's 26-year-old brother, charged with being an accessory; and Cornelius Clark, 28, an alleged passenger in the suspects' car, for obstruction of justice.

All were due in court Monday.

A memorial for the slain officers was also planned for Monday.

The Bankses' mother, Mary Smith, said she believed Marvin Banks was guilty, because he'd been suffering from drug addiction and was acting strangely.

"Yeah, he did it," Smith said.

Asked how she knew, Smith responded, "I don't know. He's out of his mind. He was out of his mind. He wasn't the same Marvin."

Alonte Tate, a sister of Liquori Tate, called her brother a hero. "He backed up his partner," she said.

Tate's family said he'd dreamed of being a police officer since he was 4. He graduated the police academy less than a year ago.

Deen was a K-9 handler and a married father of two. Hattiesburg Mayor Johnny DuPree said he didn't think the officers were targeted.

“I think we had police officers in the wrong place at the wrong time,” DuPree said. "And someone who didn’t want to go to jail, would rather shoot his way out, that’s a sad commentary."

Attorney General Loretta Lynch said Monday that the Justice Department would help local authorities investigate the killing if asked.

"The murder of these young men is a devastating reminder that the work our brave police officers perform every day is extremely dangerous, profoundly heroic, and deeply deserving of our unequivocal support," Lynch said in a statement. "All Americans owe these courageous citizens a debt of gratitude."

IN-DEPTH

Gabe Gutierrez and Jon Schuppe