The reclassification came after an FBI dive team failed to find Lyric Cook-Morrissey, 10, and Elizabeth Collins, 8, in 26-acre Meyers Lake, The Des Moines Register reported.
Rick Abben, a Black Hawk County sheriff's deputy serving as spokesman for investigators, said evidence obtained since the girls' July 13 disappearance in Evansdale was sent to the Iowa Division of Criminal Investigation's crime lab in Ankeny for analysis. The deputy declined to provide details about what was sent to the crime lab, but said no weapons had been found.
"Investigators are confident the girls are not in Meyers Lake and this case is being handled as an abduction," Abben said. "We are asking for 100 percent cooperation of everyone involved in the case.
"The girls were not in that lake. If they were lost in that area, they would have been found by now."
The girls' bicycles were found on a recreational trail near the lake.
Abben said no one has been ruled out as the abductor.
"Everyone is a suspect until we find these young girls," he said.
Checkpoints were set up in Evansdale Friday, with authorities searching vehicles and questioning drivers. Abben said house-to-house searches are possible this weekend.
A $50,000 reward was being offered for information leading to the arrest and conviction of anyone involved with the abduction of the girls.
The FBI had dispatched a California dive team to Evansdale to search the waters of the drained lake for the girls, The (Eastern Iowa) Gazette reported Friday.
City workers had started draining the lake Monday after firefighters conducted dragging operations by boats during the weekend.
Sandy Breault, a spokeswoman from FBI office in Omaha, said dogs indicated the girls' scent was in the area near Meyers Lake.
"The scent did lead to the water, which is also where their bikes were found," she said.
Family members think the girls were abducted because "they don't come here and swim here," Misty Cook-Morrissey, Lyric's mother, told CNN. "Because we haven't found them anywhere in the surrounding area, I feel like maybe they were taken."
Authorities searched the home of Cook-Morrissey and Wylma Cook, a grandmother to both girls, with the women's consent, CNN reported. Investigators seized Cook-Morrissey's computers with her consent.
Daniel Morrissey, Lyric's father, said he ended a police polygraph session this week after investigators accused him of injuring the girls and knowing where they are.
Cook-Morrissey said everything investigators were doing was difficult but necessary, CNN reported.
"We know that it's a necessary measure they have to take to get as much information as they can and of course rule us out, so we did what we have to do," she said.