LAKELAND, Fla. – A 31-year-old Florida man fatally shot an off-duty firefighter who had recently begun dating the suspect's ex-girlfriend, Polk County Sheriff's Office detectives said.

Caleb McKinney was arrested Wednesday night after he waited for ex-girlfriend, Kayla Stayner, and her new boyfriend, Clifford Cofer II, to arrive home, and then opened fire as they pulled into the driveway, homicide detectives said.

Cofer, who was sitting in the front passenger seat, died at the scene. He was a 27-year-old Sumter County fire lieutenant.

Stayner and her 5-year-old son, who was sitting directly behind Cofer during the gunfire, were not injured.

Detectives said McKinney had been distraught about his breakup with Stayner and was hospitalized in July under Florida's Baker Act law after threatening to harm himself and overdosing on pills.

Stayner, 27, of Lakeland, is the mother of McKinney's infant son. Investigators said the estranged couple had maintained contact with each other because of the child.

Detectives said Stayner made arrangements with McKinney to bring him medicine for their son, and she told him that Cofer would be with her.

Investigators said McKinney became upset, loaded his gun and put it in his pocket. After Stayner arrived and got out of the Nissan Rogue, McKinney ran across the yard and started shooting at the windshield, firing five shots.

Detectives said Stayner began yelling that her son was in the SUV, at which time McKinney pointed the gun at her and threatened to kill her.

McKinney ended up going back inside his home. McKinney's neighbor and Stayner got Stayner's son out of the SUV and called authorities.

A deputy was able to talk to McKinney on a cellphone and convince him to come out.

Investigators said McKinney, who had previously never been arrested, admitted to the crime.

McKinney faces multiple charges, including first-degree murder, attempted first-degree murder, aggravated assault with a firearm and shooting into an unoccupied conveyance. He is being held without bond.