Story highlights

NEW: "Those girls are not in that lake," sheriff's deputy says

The girls were last seen July 13, their mothers say

Dad walks out of a polygraph session, says police accuse him of hurting girl

Police search mom's home and confiscate computers with her consent

(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 8-year-old cousin Elizabeth Collins were last seen by their grandmother on Friday when they left on a bike ride. The girls’ bicycles and a purse were found near Meyers Lake hours after they were reported missing.

Grandmother: ‘We will fight’

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.

Meanwhile, authorities have searched the home of Cook-Morrissey and Wylma Cook, who is a grandmother to both girls, with the women’s consent, they told CNN. The two women said they also provided investigators with a key to get into the attic.

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, Cook-Morrisey told CNN.

Family of missing Iowa girls ‘bracing for the worst, but hoping for the best’

Also, Daniel Morrisey, Lyric’s father, told CNN that 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. The side-scanning and 360-degree sonar being brought in by the FBI needs at least 6 feet of water to function, Breault said.

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.”

Mom thinks missing girls may have been ‘taken’

And Lyric’s grandmother, Wylma Cook, said she doesn’t believe that the girls intended to go swimming.

The family has been questioned and 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.

Dogs searching for Iowa girls stopped at edge of lake

CNN’s Jim Spellman and Tristan Smith, who are both in Iowa, and Michael Martinez contributed to this report.