Evansdale, Iowa (CNN) -- A week after two young cousins went missing near a lake in Iowa, their disappearance is now being called an abduction, authorities said Friday.
A search of a 25-acre lake in the area where the girls were last seen turned up nothing, Black Hawk County Sheriff's Deputy Rick Abben told reporters.
"Those girls are not in that lake," Abben said. "Obviously, within a week if they were lost they would have been found by now. Since we can't find them (and) they're not in the lake, we're calling it an abduction."
Lyric Cook, 10, and her cousin Elizabeth Collins, 8, were last seen by their grandmother on July 13 when they left on a bike ride.
Their bicycles and a purse were found near Meyers Lake hours after they were reported missing.
Scent dogs following the girls' trail led searchers around the lake and stopped at the water's edge, according to Lyric's mother, Misty Cook-Morrissey.
Authorities have searched the home of Cook-Morrissey and Wylma Cook, who is a grandmother to both girls, with the women's consent.
The two women said they also provided investigators with a key to get into the attic.
Investigators confiscated Cook-Morrisey's computers with her consent, she said.
Daniel Morrisey, Lyric's father, said he walked out of a police polygraph session this week because investigators accused him of hurting the girls and knowing where they are.
Officials began draining the lake on Monday and the FBI was involved in the search.
But Cook-Morrissey said earlier that she thought it unlikely the girls would be found there, suggesting they might have been kidnapped.
"They don't swim there. My daughter is familiar with swimming in lakes, so I don't think she would be scared of this lake, but they don't come here and swim here," she said. "Because we haven't found them anywhere in the surrounding area, I feel like maybe they were taken."
And Lyric's grandmother, Wylma Cook, said she doesn't believe that the girls intended to go swimming.
The family has been questioned, polygraphed and had information taken from their cell phones, a process Cook-Morrissey said is hard but necessary.
"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.
CNN's Jim Spellman and Tristan Smith, who are both in Iowa, and Michael Martinez contributed to this report.