Capt. Rick Abben, spokesman for the Black Hawk County Sheriff's Office, said the silence of Dan Morrissey and Misty Cook-Morrissey is not helpful, ABC News reported.
"It's a distraction for us, and why they wish to do that, I don't know," Abben said. "We're asking for 100 percent cooperation. Why they don't want to do that, I have no idea."
Upon advice from their lawyer, the couple said they will no longer allow police to question them about the July 13 disappearance of their daughter, Lyric Cook-Morrissey, 10, and her cousin, Elizabeth Collins, 8.
The couple, who are separated, both have past convictions for felony drug offenses and a violent past that includes a restraining order, ABC News reported.
The two girls disappeared July 13 in Evansdale after going for a bike ride at Meyers Lake. Authorities found the girls' bikes along a trail and determined they had been abducted but have reason to believe they are still alive.
The FBI has asked anyone at the lake the day of the disappearance to get in touch with authorities, even if they believe they didn't see anything of importance, The Des Moines Register reported.
Notable deaths of 2014 [PHOTOS]
White House intruder had a knife