A seriously ill 9-month-old boy whose mother took him from a Seattle hospital early Thursday was the subject of an Amber Alert as police tried to find him.

Riley Rogers had been scheduled for surgery at Children's Hospital and Regional Medical Center Thursday to treat what police described as a life-threatening illness.

"The child will die imminently if he's not treated," Seattle police spokesman Sean Whitcomb said.

Police said the baby's mother, Tina Carlsen, was driving a 1994 green Plymouth Voyager van, license number 270-SVS.

The Amber Alert, which triggers public announcements on local media and also on electronic highway signs, was issued in Washington, but authorities also requested alerts in Oregon and California.

Carlsen concealed the baby and left the hospital about 6 a.m. Thursday, Whitcomb said.

Police said Carlsen does not have legal custody of her son, so she had no right to take him.

The national Amber Alert system for children who may have been abducted and are in imminent danger is named for 9-year-old Amber Hagerman of Arlington, Texas, who was killed after being kidnapped while riding her bicycle near her home in 1996.