Officers Benjamin Deen, 34, and Liquori Tate, 24
, were making a traffic stop Saturday evening when they were shot, Mayor Johnny DuPree said. They were taken to a hospital, but did not survive.
Authorities accuse the suspects of fleeing the crime scene, allegedly stealing a police cruiser and using it as a getaway car.
Joanie Calloway, 22, was charged with two counts of capital murder, the Mississippi Bureau of Investigation said Sunday.
Marvin Banks, 29, also faces two counts of capital murder, along with counts of grand theft auto and felon in possession of a firearm. Police charged his brother, Curtis Banks, with two counts of accessory after the fact of capital murder, the agency said. And a fourth suspect, Cornelius Clark, was charged with obstruction of justice Sunday, the mayor's office said.
As deputies escorted him into a police station Sunday, Curtis Banks wailed and repeated "I didn't do it."
A community mourns
Dean's and Tate's bodies have been taken to the state medical examiner's office for autopsies, while the suspects were taken "to undisclosed jails outside Forrest County to await their initial appearances" in the Forrest County Justice Court on Monday, state police said.
A vigil is planned for the officers Monday afternoon, DuPree said.
"We're a community. We're family," he said. "And we're going to grieve together through this."
The shooting marked the first time in 30 years that an officer was killed in the line of duty in Hattiesburg, the mayor told CNN.
"The men and women who go out every day to make sure that we are safe were turned on," DuPree said.
Mississippi Gov. Phil Bryant said he and his wife, Deborah, were mourning the officers' deaths.
"This should remind us to thank all law enforcement for their unwavering service to protect and serve. May God keep them all in the hollow of his hand," he said in a statement.
Slain officer had 'fire in his soul' for police work
Deen, who DuPree said was married with two children, was a K-9 officer who earned his department's officer of the year honors in 2012, according to The Clarion-Ledger
in Jackson, Mississippi.
Tate graduated last June from the police academy. His excitement at joining the force was clear as he trumpeted the news on Facebook.
"I am now a Police Officer. I would like to thank God, the Police Academy, the Police Department, my family, friends, and love ones," he wrote
He'd worked at auto parts stores for years and was thrilled to have found his calling as an officer, his father, Ronald Tate, told CNN
"He had this enthusiasm, this fire in his soul," his father recalled.
That doesn't mean he didn't realize he was putting his life in danger by putting on a police uniform.
"He really knew the risk. But I think my son just thought people are generally good, and that's just the way he was," Ronald Tate said. "He thought people are generally good people, so let's treat them all with dignity."
In addition for his excitement for his police work, Liquori Tate often shared photos on Facebook of himself posing with family members and bantered with his father over his posts.
The two talked on the phone every week and texted every day, Ronald Tate said.
In a Facebook exchange with his son in March
, Ronald Tate wrote, "Hey sonnyboi! Call ya dad sometimes! I need a good laugh!" It was a wish his son didn't take long to grant. About an hour later, Liquori Tate replied, "Calling you now lol."
On Facebook, where his timeline had been filled with condolences, Ronald Tate wrote that he'd been in a "dizzy haze" since 10:11 p.m. Saturday.
"My heart has been ripped out of my chest, and torn into a million pieces," he said. "Gotta get down to MS where my daughter is. She's absolutely devastated. He was clearly her protector, and friend."