The twins, who are from Dracut, have injuries consistent with shaken baby syndrome, a state official said, and the boy's condition was listed as critical last night.

"It's a horrendous situation," said Dr. Brian Gilchrist, chief of pediatric surgery at the Floating Hospital and one of the physicians handling the case. "It has shaken up my hospital."

The parents and other caretakers of the twins are being questioned by police and prosecutors, though no charges have been filed, said Emily LaGrassa, spokeswoman for the Middlesex district attorney's office.

Gilchrist described the boy as having "the most beautiful almond-shaped eyes you've ever seen." He said patient confidentiality rules prohibited him from saying anything else about the boy's condition.

"I'm sick over it," Gilchrist said. "Who could do this to a child?"

Emergency personnel responded to an apartment on Frederick Street in Dracut Monday morning, after receiving a 911 call that "a baby had stopped breathing," said Dracut Police Chief Kevin Richardson. The boy was taken to Lowell General Hospital and then flown to the Floating Hospital at Tufts-New England Medical Center, where he remained last night.

Alarmed by the boy's injuries, investigators removed his twin sister from the Dracut home and brought her for examination to the Floating Hospital. The girl was found to have suffered similar, but less severe injuries. She was released from the hospital yesterday and was moved to a foster home with medical expertise, said Denise Monteiro, spokeswoman for the state Department of Social Services.

"Both of them had injuries that are consistent with shaken baby syndrome," Monteiro said.

Monteiro and Dracut officials declined to say who alerted authorities to the boy's injuries, and the names of the parents and other relatives are being withheld while the investigation continues.

She said the mother of the twins, who is in her mid-20s, does not have other children and had no previous record of child abuse or neglect. Monteiro said the father also has no previous history with her agency.

Chad Woods, 35, lives at the apartment building in Dracut where police and ambulance crews arrived Monday morning. He said the twins' mother, who lives there with her mother and sister, was "very responsible and very loving of her kids." He said he did not know the twins' father.

"I pray for the kids," he said.

The typical case of shaken baby syndrome involves an infant who has been shaken so vigorously that the brain moves back and forth inside the skull, rupturing blood vessels and damaging brain tissue . The violent shaking can fracture bones. Most often, the perpetrators are caretakers who say they shook the baby after being overwhelmed by the child's crying. Educating parents and other caretakers about the rigors of caring for babies, as well as their physical vulnerabilities , has been the centerpiece of nationwide efforts to reduce shaken baby syndrome.

A month ago, Governor Mitt Romney signed legislation to protect infants and children from shaken baby syndrome, which is the leading cause of injury-related infant deaths nationwide. Nearly one out of every three children who suffered such shaking ultimately die from the injuries, and those who survive often have lifelong physical and mental impairment , according to state officials.

In Massachusetts, state child-protection officials confirmed 76 cases of shaken baby syndrome from 2001 to 2004. Of those cases, nearly half were shaken by their biological parents.