She said she doesn't believe Sean Michael Phillips, the baby's father and her alleged abducter, gave their daughter to anyone else.
"I think if anything, he just left her somewhere," she said. "Nobody's getting anything out of him now."
Courtland spoke to reporters briefly before leaving her apartment, 922 E. Tinkham, to look for Kate. She said she wasn't sure where to look, but would start with places Phillips was likely to go.
Courtland said she and Phillips had argued before Phillips allegedly took Kate, but she never suspected that he might harm their daughter.
"We'd argued before. We're just like other parents," she said. "I never had an indication that he would take a child."
She said police told her not to discuss what happened in more detail, and neighbors said they didn't know the couple.
The two have an older daughter, Courtland said, who wants to know where her father and little sister are.
"I just tell her that Kate's lost and mom and everybody else is trying to find her," she said.
Courtland said she has already taken a polygraph test and passed.
"I don't want people to ever, for one second, think that I would harm my child," she said.
The frantic search for the missing baby now involves federal and state authorities, including the U.S. Coast Guard, Ludington Police said.
The Ludington Daily News reported that authorities are also looking for people that were seen launching a blue-and-white-colored pleasure boat at the Hackert Lake public boat launch Wednesday afternoon.
Hackert Lake is also known as Crystal Lake. The boaters were launching the boat with a silver-colored four-door pickup truck. The potential witnesses were described to investigators as a family with children and were possibly driving a green full-size SUV as well.
Police have indicated that Phillips, 21, of Scottville, took the baby. He has since been located by authorities and lodged in the Mason County Jail. But the baby has not been found. Phillips' vehicle, a silver, 1998 Oldsmobile Intrigue, is currently in police custody.