Search fails to turn up missing Iowa girls; 'It's like they vanished,' official says July 16, 2012 -- Updated 0912 GMT (1712 HKT) Elizabeth Collins, 8, and Lyric Cook, 10, have been missing since Friday The girls, who are also cousins, were last seen on their bicycles in Evansdale, Iowa A massive volunteer search effort grew to over 900 people over the weekend The FBI and the Iowa Department of Criminal Investigation have joined the search (CNN) -- As the search moved into its fourth day Monday for two girls last seen riding their bicycles near a lake in Evansdale, Iowa, authorities suspended a volunteer effort that drew hundreds over the weekend. The girls -- 8-year-old Elizabeth Collins and 10-year-old Lyric Cook -- were last seen by their grandmother on Friday when the two, who are cousins, left her home to go ride their bicycles, authorities said. A massive volunteer search effort that grew to over 900 people by Sunday failed to locate the girls. "As we evaluate the accomplishments and results of the searches conducted this past weekend, we are not asking for any additional assistance from the general public," Evansdale Police Chief Kent Smock said in an e-mail early Monday sent to news outlets. He said the volunteer effort was being suspended for now. There are more questions than answers about what happened to the girls, whose bicycles were located hours after they were reported missing near Meyers Lake. Since then, authorities say, little evidence has turned up. "It's like they vanished," Richard Abben of the Black Hawk County Sheriff's Office told CNN affiliate KWWL. There's just nothing. The search for the girls remains a missing persons case. "We have no indication of foul play," Abben said. Agents from the FBI and the Iowa Department of Criminal Investigation joined the search on Saturday, authorities said. CNN's Leslie Tripp contributed to this report.