(CNN) -- Three small children -- all apparently 3 years old or younger and with no parent in sight -- were found Thursday among a group of homeless people in Portland, Oregon, police said.

The youngsters' identities are unknown. Portland police issued a statement saying its officers are working with the state Department of Human Services to learn more about the children, most importantly who they are.

The investigation began with a 911 call, placed around 9:25 a.m. (12:25 p.m. ET) Thursday, about a man who'd heard children's voices apparently emanating from a shed behind his house in East Portland.

Police officers arrived and found the "vacant residence" where the sounds had come from. There they talked to "a number of homeless people on the premises" and discovered the three "olive-skinned" children.

The youngest among them is described by police as a 15-pound infant who is between the age of 8 and 15 months old. She has short black hair, brown eyes and two front, lower teeth showing.

The next oldest child found is another girl. Weighing 28 pounds, she has long black hair and brown eyes and is about 2 years old, police estimate.

The third child is a boy, approximately 3 years old. He also weighs 28 pounds and has brown eyes, as well as short black hair.

Homeless people told police that a woman, believed to be the three's mother, had abandoned the children there the night before.

Authorities are pleading for the public's help to identity the children, who are currently in protective custody and being medically evaluated.

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