Seventh-grader Isabella Oleschuk was reported missing Sunday morning.
"We are not going to stop until we find Isabella," Orange police Chief Robert Gagne said at news conference. "My hope is, when this is all over, we will find her safe."
Gagne declined to say whether authorities suspected foul play.
A girl matching Isabella's description was seen Sunday morning walking and holding hands with a boy slightly taller than her at a nearby pond, Gagne said. He said the jogger who saw them did not notice any signs of distress. Authorities asked anyone else who saw the pair to call police at 203-891-2130.
The girl is 5 feet 4 inches tall, weighs 120 pounds and has blue eyes and shoulder-length blond hair. She may be wearing a light-blue jacket, brown-felt hooded cape and black rain boots.
A vigil was planned Monday night to pray for Isabella. The Rev. Tod King, a pastor at the Orange Congregational Church, said Isabella is "a delightful child" who is quiet and bright and loves animals and nature.
Gagne said the two-day search has involved dogs as well as an FBI helicopter and an airplane. Crews also have searched the woods behind the girl's house.