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.