Waterville, Maine (CNN) -- Divers in Maine were scouring a river Wednesday in their search for a missing 21-month-old girl, but had found no sign of her in the frigid waters, authorities said.





"We're making progress. I can tell you honestly that as the days go on, our concern grows, as it's been 26 days. But I can also tell you that we remain hopeful that we will bring her back to her home," Maine Department of Public Safety spokesman Steve McCausland told reporters.





The search for Ayla Reynolds focused Wednesday on a half-mile area of the Kennebec River that stretches from downtown Waterville to the Carter Memorial Bridge. Officials said they had received more than 600 tips, but nothing that specifically pointed to that area.





The river still has open water despite air temperatures in the teens.





Divers in special suits to protect them in the frigid waters rotated in and out of the river, which was around freezing temperature, Maine Warden Service Lt. Kevin Adam said.





"The water is clear. When the sun is out, they can see 4-6 feet. ... We picked today because the weather's good and it was the most efficient time to do it," he said.





The search for the little girl is in its fourth week. Police have said they suspect foul play in the case.





"All we know is that Ayla's missing. We've ruled out no scenario. We've ruled out no one. And we're not going to get into investigative details," McCausland said Wednesday.





He described the search as the most intense investigation Maine's state police had been involved in for two decades.





Ayla's parents are not married and don't live together. Her mother recently completed rehab, family members said. Investigators said both families have cooperated with police.





The little girl's father, Justin DiPietro, said he put his daughter to bed at 8 p.m. December 16 and called police the next morning just before 9 a.m. to report her missing, according to authorities.





DiPietro's mother, Phoebe DiPietro, told CNN she wasn't at the home the night Ayla disappeared. But she said she fully supports her son, who has said he "would never do anything to harm" his daughter.





She said she just wants her Ayla returned unharmed.





"I'd give everything I owned if we can have her back," she said last week, her voice breaking.





CNN's Susan Candiotti and Ross Levitt contributed to this report.
Who said their concern grows? When has she been missing since? Who was Ayla Reynolds? What did a police spokesman say? Who is missing? What did the divers have to do? Who has been missing?
Maine Department of Public Safety spokesman Steve McCausland 26 days. 21-month-old girl, "All we know is that Ayla's missing. We've ruled out no scenario. We've ruled out no one. And we're not going to get into investigative details," Ayla Reynolds scouring a river Ayla Reynolds
question: Who said their concern grows?, answer: Maine Department of Public Safety spokesman Steve McCausland | question: When has she been missing since?, answer: 26 days. | question: Who was Ayla Reynolds?, answer: 21-month-old girl, | question: What did a police spokesman say?, answer: "All we know is that Ayla's missing. We've ruled out no scenario. We've ruled out no one. And we're not going to get into investigative details," | question: Who is missing?, answer: Ayla Reynolds | question: What did the divers have to do?, answer: scouring a river | question: Who has been missing?, answer: Ayla Reynolds