(ABC News) -- Police searching for two Iowa girls who haven't been seen for three days said they had no clue as to what might have happened to them.
Elizabeth Collins, 8, and her cousin, Lyric Cook, 10, were last seen around midday Friday, riding their bicycles in downtown Evansdale.
Police, now way past the crucial 24-hour window period, are working with federal authorities on what to do next.
"Every hour, you know, it makes it that much worse. The first 24 hours of course are very critical. We've obviously surpassed that now by quite a bit," said Capt. Rick Abben, a chief deputy with the Black Hawk County Sheriff's Office.
As of Sunday evening, authorities had not decided about issuing another call for volunteers to search Monday. Authorities do plan a news briefing late Monday morning to include any overnight developments, according to ABC News station KCRG.
Collins and Cook vanished from Cook's grandmother's home around noon, and were reported missing about three hours later.
The same day the girls disappeared, police found their bicycles and a bag they were carrying on a trail near Meyers Lake. Crews used boats to search the lake, and volunteers looked in the woods Saturday and Sunday but haven't found any clues.
No Amber alert was issued in the case, because it does not meet the requirements, Abben said. "Because we have no person that was seen and we have no vehicle that was described, so we can't issue an Amber alert by their guidelines," Abben said.
The girls' grandmother, Wylma Cook, says she fears they were abducted.
"Whoever has them, just turn them in, let them loose anywhere so they can call me," Cook said. "Lyric knows my cell phone, she knows my house phone.
Tammy Brousseau, Cook's aunt, says she knew what to do if a stranger approached her.
"I taught her myself if they got a hold of your arm, drop to the ground, kick, bite, scream, do everything you can," said Brousseau.
More than 1,000 volunteers have fanned out around the area to look for the girls. The response was so overwhelming that when hundreds of volunteers showed up Sunday, search organizers said it took several hours to bus people to assigned areas. Neighbors in this small, tight-knit community say it's the least they could do to help.
"I have two little girls at home. Just want to bring them home safe to their mommy and daddy," said volunteer Kim Einfelt. "Can't lose hope. Can't."
ABC News' Olivia Katrandjian and ABC News Radio contributed to this report.