(CNN) -- A 25-year-old Oklahoma man was charged Friday with murder in the deaths of two girls who were found about three and a half years ago, shot to death in a ditch alongside a remote country road.
"We don't believe that he knew (the slain girls) directly," Oklahoma State Bureau of Investigation Director Stan Florence said Friday of Kevin Sweat. "We just believe that he happened to be in the area that day."
According to a probable cause affidavit, Sweat told investigators on September 13 that he'd been driving his Chevrolet Cavalier where the girls were found when he saw "two monsters" come at him.
The suspect said he "panicked," shooting the "monsters" first with a Glock .40 handgun and then with a .22 handgun he'd had in his glove box, the affidavit said. It claimed Sweat had voluntarily waived his rights and agreed to talk to investigators.
The "monsters" are believed to be best friends Taylor Paschal-Placker, 13, of Weleetka and Skyla Jade Whittaker, 11, of nearby Henryetta, who were discovered June 8, 2008, by Taylor's grandfather. They had been shot in the head and chest, eight times for Skyla and five for Taylor, the state medical examiner reported after an autopsy.
Their killings rattled Weleetka, a town of just over 1,000 residents, with police calling the shootings the community's first murders in more than 20 years.
They also set off an extensive investigation involving multiple local, state and federal law enforcement agencies and including about 650 interviews, 900 leads and 19,000 forensic tests on 800 pieces of evidence, according to Florence.
In January 2010, Sweat was among those interviewed about the girls' murder, because he owned a Glock .40 Model 22 handgun like one of those thought to be used in the shootings. He then told an investigator then that he'd sold such a gun in 2007, according to the arrest affidavit.
But he was not considered a suspect until this year, after being arrested and charged with the murder of his girlfriend, Ashley Taylor, Florence said.
He was already in the Seminole County Jail in his girlfriend's death when the new charges were filed, according to the state investigative bureau director.
He is charged on four counts related to the 2008 case: two each of first-degree murder, plus two each of shooting with intent to kill, according to the affidavit. The witnesses cited in this document include several of Sweat's family members, including his father, as well as the two victims' relatives and the Glock gun company.
District Attorney Max Cook, whose jurisdiction includes Creek and Okfuskee counties, said Friday that he has filed court documents requesting that Sweat be eligible for the death penalty if found guilty of murder in any of these killings.
"We feel that we are in an appropriate position to go forward in this case," Cook said Friday, referring to the case of the two girls.
Neighbors and relatives described Taylor and Skyla as nearly inseparable, often playing together after school, riding their bikes and sleeping at each others' houses. They were at Taylor's house the night of June 8, when they set off about 5 p.m. for a walk down the road.
Soon thereafter, Taylor's grandfather made a futile attempt to call her on her cell phone. He found the two girls about 30 minutes later, lying side-by-side in a ditch about a quarter-mile from the home, police said.
Six weeks after the slayings, Oklahoma authorities released a 911 tape in which a breathless, nearly hysterical woman -- identified only as a family member of one of the victims -- is heard screaming, "Somebody killed two girls."
"They went for a walk, and now they are both down here dead. ... My granddaughter and her friend," the woman said on the tape. "Help me. Please!"
On September 30, a $5,000 reward was offered for information on "the Glock .40 model 22 handgun used in the Weleetka girls' homicides," the state investigation bureau noted on its website. A serial number was given for that weapon, one of two linked to the girls' deaths.
Ballistics tests determined that the .40-caliber shell casings found at the crime scene, matched those found on the property of Curtis Sweat, Kevin's father, the affidavit said.
Investigators determined the serial number of the gun, which had been sold to Sweat. In his September interview, the suspect admitted that he'd purchased the Glock .40 gun in 2007.
Authorities on Friday reiterated their plea for the public's help in tracking down this firearm, which they said Sweat may have tried to sell in March at a Tulsa gun show.
Two photos of the suspect were also released: one taken around the time of the Weleetka killings and the other a mug shot after he was charged in his girlfriend's death.
"We hope that, by releasing (those photos), it will spark someone's memory of seeing Mr. Sweat around the scene of (Taylor and Skyla's) murder," Florence said.