ST. LOUIS (AP) -- While investigators tried to unravel the story of a 16-year-old boy who weighed just 40 pounds when he was taken to an Illinois hospital this week, a few details about his background were emerging.

The boy from Michigan -- who is mentally retarded, prone to seizures and has cerebral palsy -- was in serious condition in the intensive care unit at Cardinal Glennon Children's Hospital in St. Louis.

Investigators said they had indications that abuse of the boy dates back to when he was shaken as a baby in San Jose, Calif.

State investigators said the boy tested positive for drugs and that given his condition he could not have taken them without assistance.

At a hearing Thursday in St. Louis Family Court, an advocate for the boy said he was severely malnourished and dehydrated, and that doctors found marijuana and methamphetamine in his system, KMOV-TV reported. The judge ordered that he be placed in protective custody.

Jill Manuel, a spokeswoman for the Illinois Department of Children and Family Services, said the boy was traveling through Illinois on Monday with his mother and a 14-year-old brother. The mother was apparently homeless, and had recently been in Texas, Illinois and Indiana.

Investigators are looking into possible prior cases of abuse -- the boy previously was in protective custody in Michigan.

At the hearing, investigators said that the boy's father is a convicted sex abuser who cannot legally have contact with him, the St. Louis Post-Dispatch reported.

It was not clear what prompted the decision to take the boy to a hospital in Mount Vernon, Ill.

"Obviously the child probably was in some distress, but I don't know really what triggered it," said Manuel. She said that officials at the Illinois hospital decided it would be better to have him treated at one in St. Louis, and he was flown there.

Spokeswoman Deb Hendricks of the Children's Division of the Missouri Department of Social Services said the case involves several states, making it a more involved investigation and more difficult to determine where the teen should be placed.

"We're still investigating this," Hendricks said. "We don't have all of the facts. Our focus really is just on the boy and his well-being."

Ann Ricci, the emergency room physician who treated the boy at Cardinal Glennon, would not discuss the case, but said a healthy 16-year-old should weigh between 120 to 170 pounds and that 40 pounds was an alarmingly low weight.