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 do n'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 was n'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 .
NEW : Police spokesman : `` As the days go on , our concern grows '' NEW : `` We 've ruled out no scenario . The only thing that's possible is a terrorist attack in which case we will be there to protect you and your family from any harm or danger. Divers wearing special suits brave frigid temperatures . Ayla Reynolds has been missing since mid-December .