(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.
"It's like they vanished," Abben told CNN affiliate KWWL. "There's just nothing."