EVANSDALE, Iowa (CNN) — A week after two young cousins went missing near a lake in Iowa, the FBI is set to deploy divers and sonar Friday in an attempt to find them, an agency official said.
The FBI dive team will employ two types of sonar in the search at Meyers Lake, spokeswoman Sandy Breault said Thursday.
Authorities drained about half of the 25-acre lake Thursday when the pumping operation was halted after three days, city officials said.
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.
Meanwhile, 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. The FBI is bringing a side-scanning and 360-degree sonar that needs at least 6 feet of water to function.
Divers were expected to go into the water Friday, she said. But Cook-Morrissey said she thinks it’s unlikely the girls will 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.
The small community of population 4,751 in northeast Iowa has never suffered through anything like the worrying going on now, the mayor said.
The search remains a missing persons case, authorities said.
CNN’s Jim Spellman and Tristan Smith, who are both in Iowa, and Michael Martinez contributed to this report.