A man and woman in Mississippi were arrested and charged with murder on Sunday after two police officers were shot dead a day earlier while conducting a routine traffic stop in Hattiesburg, officials said.
Authorities had launched a manhunt after at least one suspect fled the scene of the shooting in a police car later found abandoned, and residents in the city about 130km southeast of Jackson were told to stay indoors.
Marvin Banks, 29, and Joanie Calloway, 22, were each charged with two counts of capital murder, police said.
Banks' brother Curtis, 26, was also arrested and charged with two counts of accessory after the fact of capital murder.
Police said a fourth man, identified as Cornelius Clark, was arrested later on Sunday in connection with the case and booked on an obstruction of justice charge.
DuPree said members of the public quickly helped bring to justice those behind the first killing of a police officer in the city for more than 30 years.
"The community helped us," he said, without elaborating. "No parent wants to bury their child before them. And here we are on Mothers' Day. It's a tragedy for all America," he said.
The shooting happened on Saturday evening after Officer Benjamin Deen, 34, stopped a gold Cadillac Escalade, and Liquori Tate, a 25-year-old recent police academy graduate, arrived as backup.
Two civilians called for help after finding the wounded officers. They were pronounced dead at a hospital in Hattiesburg.
DuPree told a news conference it was too early to discuss a possible motive, and that a probe by the Mississippi Bureau of Investigation was ongoing.
"We know the officer called for backup for a reason … we'll find that out," he said. "We will not jeopardise this investigation by guessing."
State court records showed the Banks brothers are felons with multiple arrests for firearms charges.
DuPree said Calloway had been driving the Escalade at the time of the shooting. The suspects were arrested in different locations in Hattiesburg, police said.
Deen and Tate are the ninth and 10th law enforcement officers in the United States shot and killed by assailants in 2015, according to the Officer Down Memorial Page, a non-profit organisation that tracks police deaths in the line of duty.