Preliminary autopsies on the dead children appear to show they were drowned, Ace Hart, a deputy St Clair County coroner said.
The autopsies showed no signs of physical abuse or trauma on the children - aged seven, two and one - and toxicology tests were pending to see if they were poisoned or possibly drugged, Mr Hart said.
Tiffany Hall, 24, has not yet been charged in the children's deaths, but prosecutors have accused her of killing their mother, Jimella Tunstall, 23, and her foetus. The foetus had been cut from her womb, authorities said.
Authorities suspect Ms Tunstall was slain on or about September 15. That day, Hall summoned police to a park, saying she had given birth to a stillborn child, Mr Hart said.
She was arrested after she told her boyfriend during the baby's funeral that the baby was not his and that she had killed the mother to get it, authorities said.
Ms Tunstall's body was found Thursday, and authorities began a furious search for her other children. Police said the children were last seen with Hall on Monday.
Authorities had visited Ms Tunstall's apartment but noticed nothing amiss while looking for photographs of the children for media outlets to publicise the search, Mr Hart said.
Hall told police where the bodies were and told police she drowned them, he added.
Children overlooked
Mr Hart said he understood why investigators may have overlooked the children during their previous trip to the apartment. "Who would be looking in the washer and dryer?"
But by Saturday night "you could find them by the smell", Mr Hart said.
The oldest, seven-year-old DeMond Tunstall, was found in the dryer and the younger two children - two-year-old Ivan Tunstall-Collins and one-year-old Jinela Tunstall - in the washer.
Two of the children were found nude, the third wearing only underpants.
Hall remains in jail on a US$5 million (A$6.61 million) bond, charged with first-degree murder in Tunstall's death and with intentional homicide of an unborn child.
An autopsy showed that Jimella Tunstall bled to death after sustaining an abdominal wound caused by a sharp object, believed to be scissors.
Authorities believe her womb was cut open after she was knocked unconscious.
Relatives say Tunstall grew up with Hall and had let her baby-sit her children. Hall has two children of her own.