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(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.

Abben said there was nothing to indicate problems with "custodial issues."

"We've talked with the parents and they've been cooperative," he told CNN affiliate KGAN.

The family now wonders if the girls were abducted, said Tammy Brousseau, an aunt to both girls.

"In the area where the bikes were found there is a very brush area where somebody could have pulled up -- maybe had been watching them for a little bit of time," she said.

Elizabeth's parents say it gets harder every day that passes without news of the girls.

"Positive thinking is all we have right now and our faith that God will bring them back," Elizabeth's mother, Heather Collins, told KGAN on Sunday.

The girls' grandmother, Wylma Cook, told the CNN affiliate she was the last person to see them.

"They asked me to go for a short bike ride, and I said go ahead for a short bike ride. They never came back," she said.

Authorities began dragging Meyers Lake over the weekend for a sign of the girls.

The volunteer search effort that ended Sunday night found nothing to indicate what might have happened to the cousins.

The more time that goes by for Cook, the harder it gets to remain positive.

"Time is what bothers me," Cook said. "Too long, something's not right."

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.