DES MOINES, Iowa (AP) — With the trail apparently cold in the search for two missing Iowa girls, the FBI is sending in tracking dogs and local authorities are draining a lake near where the girls’ bicycles were found.

Elizabeth Collins, 8, and her cousin Lyric Cook-Morrissey, 10, were last reported seen Friday afternoon leaving their grandmother’s house. Their bicycles and Elizabeth’s purse were found later that day near a bike trail at the edge of Meyers Lake in the northeast Iowa city of Evansdale.

What followed was a massive sweep of the area that drew hundreds of volunteers over the weekend. But authorities suspended that effort Monday, saying it had failed to produce significant leads.

Black Hawk County Chief Deputy Rick Abben said the draining of the nearly 5-acre lake began Monday and could take up to three days. Officials believe it will go faster due to the low level of the lake and the Cedar River into which it drains.

Authorities previously dredged the lake and have been interviewing family, friends and registered sex offenders who live in the area.

Abben said local, state and federal officials have been “grasping for straws” in the frantic search. A tip line turned up numerous reports of articles of clothing that had been found, but none belonged to the girls. He said it was as if they had just disappeared.

“It wouldn’t be proper for me to stand here and tell you we have a theory, because we don’t,” he said. “We have two missing girls, and we have no idea why.”

He said the decision to drain the lake was made to rule out with 100 percent certainty that the girls are not in the water. Once that’s done, all resources can be used elsewhere.

FBI spokeswoman Sandy Breault said the agency was working to get photos of the missing girls onto more billboards and websites. She said the bureau was sending in dogs trained to search for human scents in the area where the girls went missing.

Chari Paulson, assistant director of the Iowa Division of Criminal Investigation, said 15 agents had been assigned to the search for the missing girls since the agency joined the effort Saturday. She said agents are chasing any leads or information from the public and the family or searchers as they are brought to their attention.

“These are some of the most important types of cases we work, finding missing kids. We are deploying any and all resources we can to help resolve this case,” she said.

Abben said the sex offender interviews led to no new information.

“Everyone has been cleared,” Abben said. “We have no concerns with that.”

Abben said the case is still considered a missing persons investigation and there was nothing to suggest the girls left Evansdale or had been abducted.

A search of a 12-mile radius drew nearly 1,000 volunteers Saturday and almost 900 Sunday, but there were no significant leads, Abben said. With the heat and people going back to work, authorities decided not to do another large-scale search Monday.

“Right now we don’t feel putting more civilians on the street looking for us is going to be a benefit,” he said.

Instead, rescue teams searched the lake by boat again Monday morning and began the draining process in the afternoon. Officers stopped cars at a number of intersections in town, questioning some drivers and searching trunks, the Waterloo-Cedar Falls Courier reported. Others passed out fliers about the girls.

Lyric’s mother described her daughter as a good swimmer, and Elizabeth’s mother has said she didn’t think the girls would stray far from home.

Elizabeth’s parents didn’t respond to calls Monday from The Associated Press, and Lyric’s family couldn’t be reached for comment.