Elizabeth Collins, 8, and Lyric Cook-Morrissey, 10, were last seen Friday at their grandmother's house in Evansdale, a suburb of Waterloo. Their bicycles and Elizabeth's purse were found later that day at nearby Meyers Lake, and FBI dogs detected the girls' scent in the area.
FBI spokeswoman Sandy Breault said that if the divers are deployed, they would search the deeper holes of water in the lake, which may be as deep as 20 feet, she said.
Breault also says the FBI's Child Abduction Rapid Deployment team of five or six is on the scene in Iowa. She would not comment on the focus of their investigation.
Family members of the two young girls believe the cousins were abducted after days of investigations appeared to produce no significant leads.
On Monday, authorities began draining the lake, a task that they said could take up to three days.
Officers paddled around the lake in kayaks Tuesday while others walked the shoreline hunting for clues. One officer picked up something that appeared to be a cup and placed it in an evidence bag, but officials did not identify the object to the media.
Breault said the reaction from the dogs indicated a "strong possibility" the girls had been at the lake but that because there were no confirmed sightings there, authorities couldn't be certain.
Chief Deputy Rick Abben of the Black Hawk County Sheriff's Office said the case was still considered a missing persons investigation and that they were draining the lake to rule out the possibility the girls were in the water.
Lyric's parents said they didn't think the girls had drowned, noting their swimming abilities and the fact that no shoes were recovered on the bank of the lake. Relatives suspected abduction.
"I don't think that they're in the lake, not at all, but it is just like a dead end as far as we know so far," said Dan Morrissey, Lyric's father.
"The area where the bikes were found is fenced on both sides, and it is right where maintenance gate is. It is a spot that looks to me like a trap," Morrissey said. "Somebody could have just come along right then or followed them down. It would have been the worst spot to be in."
Tammy Brousseau, 48, an aunt to both girls, agreed that the girls may have been abducted. She said just last week she taught Lyric how to "save someone if they went limp in the water" and that she also taught both girls how to "drop to the ground and fight" if someone tried to snatch them.
A massive sweep of the area that drew hundreds of volunteers over the weekend failed to find any evidence. Authorities dredged the lake and interviewed family, friends and registered sex offenders who live in the area. At least 30 FBI investigators joined the search.
Abben said a tip line turned up numerous reports of articles of clothing that had been found, but none belonged to the girls. He declined to talk specifically about evidence other than saying the bicycles and purse, which had a cellphone in it, could not confirm the girls had been at the lake. The cellphone was used by the girls to play games and was not activated to make calls, he said.
He said the family has been cooperating with the investigation.
"We have no reason to believe there was any foul play at this time," he said. "Everyone is suspect until we find something."