(CNN) -- Two Oklahoma schoolgirls were killed with two guns, leading investigators to believe that more than one person was involved in their deaths, authorities said Wednesday.

Taylor Dawn Paschal-Placker, 13, of Weleetka and Skyla Jade Whitaker, 11, of Henryetta, were shot multiple times Sunday on a rural road about a quarter-mile from Taylor's home.

Investigators don't have any suspects or motives, but a forensic examination of the bodies indicated that two guns had been used, Oklahoma State Bureau of Investigations Agent Ben Rosser said Wednesday.

"This leads us to believe that we're probably looking for two shooters," Rosser said at a news conference Wednesday, where he also announced that authorities had raised the reward to $25,000. Watch what officials are doing to solve the case »

With two shooters, Rosser said he was hopeful that one would turn on the other and come forward.

Rosser said investigators were considering a variety of motives as they searched for suspects, including the possibility of a "random thrill kill."

Because of the girls' ages, Rosser said investigators were looking at computer evidence and questioning young people in the area to develop possible leads.

Authorities said Tuesday that they believed the shooters were most likely from the area, considering the remote location where the bodies were found.

The two girls had planned a sleepover at Taylor's house the night they were killed. They left the house about 5 p.m.

Less than 30 minutes later, Taylor's grandfather discovered the girls' bodies in a ditch on the side of a road, near a bridge that is a popular gathering spot in the area.

"It is an isolated area." Rosser reiterated Wednesday. "They had to give my agents explicit directions Sunday on how to get there, in broad daylight."

The killings have rattled the community of Weleetka, a town of about 1,000 residents 75 miles from Tulsa. Taylor and Skyla were the only girls in their sixth- and fifth-grade classes, according to The Oklahoman newspaper.

Taylor's uncle, Joe Mosher, described his niece as an intelligent girl who loved animals.

"She rescued turtles on the highway and wrote her name on them and turned them loose in the country," Mosher said Wednesday.

Mosher said Taylor was at the top of her class in public school after being homeschooled most of her life.

"She was very smart," said Mosher, who last saw Taylor two weeks ago at a family reunion.

In the wake of the shootings, authorities have urged parents in Weleetka to keep a close eye on their children.

"Two little girls can't go walking down a country lane," Rosser said. "I think that's pretty sad."

Police are asking members of the public with more information about the case to call (800) 522-8017.

To make a donation to a burial fund for the victims' families, the public can contact the Bank of Commerce in Weleetka, Rosser said.

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