(CNN) -- A search was in its fourth day Monday for two girls last seen riding their bicycles near a lake in Evansdale, Iowa, as 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 ride their bicycles, authorities said.
A massive volunteer search effort over the weekend 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 is suspended for now.
"We really have nothing new that I can tell you, other than the fact the search is continuing," Rick Abben, chief deputy for the Black Hawk County Sheriff's Office, told reporters Monday. "... We're still looking for both of them. There's no new evidence or anything that's been found."
Nearly 1,000 volunteers helped in the search Saturday, and nearly 900 on Sunday, he said.
Calls are coming in to a tip line, he said, and each bit of information is being checked out. Police ask that anyone who may have seen the girls on Friday contact authorities, he said.
Abben said the volunteer effort was suspended because authorities did not feel there was a benefit in having more people search. However, he said, "we can always call those folks back out."
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," Abben 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.
He said there has been nothing to indicate problems with "custodial issues."
"We've talked with the parents and they've been cooperative," he told CNN affiliate KGAN.
He told reporters Monday there was no indication the girls were outside Evansdale, a town of about 4,700 located about 12 miles southeast of Cedar Falls.
"Obviously, this is something that's very different for the community," Abben said. "... We have two missing girls, and we have no idea why."
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 brushy 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 more time that goes by, the harder it gets for Wylma Cook to remain positive, she said.
"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.