The cistern, a receptacle designed to hold rainwater and other liquid, was beneath a backyard deck at a home neighbors said had been empty for about four years.
On Monday, television stations in Kansas City have been subpoenaed to submit footage of interviews with family and friends of the missing baby.
The subpoenas ask that media present the documents on Oct. 18, but it was unclear if the television stations would comply.
Lisa Irwin disappeared last Tuesday. Her parents said she was kidnapped from their home and that a window had been tampered with the night she vanished.
Detectives searched the Irwin residence on Monday, including neighbor's yards. On Sunday they tried to reenact how an intruder could have entered the bedroom.
Federal authorities have joined the search, but so far there are no solid leads. Police have said they have no suspects in the case.
The Associated Press contributed to this report.
