One of the year-old conjoined twin girls who underwent surgery to separate their heads died shortly after the procedure was completed Thursday, a hospital spokeswoman said.

Lea and Tabea Block (search), from Lemgo, Germany, were successfully separated at 12:15 a.m. after more than 18 hours of surgery, but Tabea died later "after an exhaustive resuscitative effort," Johns Hopkins Children's Center (search) spokeswoman Staci Vernick Goldberg said.

Lea was in critical but stable condition and "doing well" in the hospital's pediatric intensive care unit, Goldberg said.

"We extend our deepest sympathy to the parents and family of Tabea Block," Goldberg said. "At the same time, we have great hope that Lea will remain strong, recover well and grow into a healthy young girl."

The girls' parents — Nelly, 27, and Peter, 28 — have not made a public statement.

The surgery to separate the girls had resumed at 6 a.m. Wednesday, four days after the operation was halted temporarily when the condition of one twin — the hospital wouldn't say which one — became unstable.

On Wednesday afternoon, nearly eight hours into the second operation, the hospital announced that doctors were halfway through a critical stage, separation of a web of blood vessels at the back of the head known as the dural venous complex.

Late Wednesday night, the hospital reported that the surgical team, led by Dr. Ben Carson, continued to "tease apart brain tissue, and separate the blood vessels that the twins still share."

Once the girls were separated, doctors had planned to reconstruct the dura (search), the tough fibrous covering of the brain, and close their scalps, the spokeswoman had said. The hospital said on its Web site that it would not provide an update on the surviving girl until later in the day.

The surgery was originally expected to last 24 to 48 hours. The twins were joined at the tops of their heads and shared several blood vessels between their brains.

Carson was part of a team last year that made the first attempt to separate adult twins joined at the head. The two Iranian women, Laleh and Ladan Bijani (search), 29, died during the three-day surgery in Singapore.

Carson also led a team in 1987 that separated 7-month-old boys from Germany, using for the first time a procedure in which their circulatory system was bypassed and their bodies were cooled to preserve brain function.

In August, Carl and Clarence Aguirre (search), 2-year-old twins from the Philippines also born joined at the head, were separated at a hospital in New York. The boys are undergoing physical therapy and other rehabilitation and are doing well, their pediatrician said last week.