LOS ANGELES (AP) — An investigation began Wednesday into why a 75-year-old, 75-foot-tall pine tree fell on a group of kids from a summer camp at a Southern California children’s museum, leaving a young boy and girl hospitalized with serious injuries, officials said.

An independent arborist and another from the city of Pasadena spent about two hours carefully examining the pine to try to find what caused it to fall a day earlier when there was a clear sky and little wind.

“They’ve inspected the soil, the bark, the tree limbs, it’s been a pretty exhaustive inspection and probing of the tree,” city spokesman William Boyer said.

There was no immediate word on what the investigation revealed.

“They’re still digesting all of the data they’ve collected,” Boyer said.

Fire Department officials said earlier that the tree had showed no signs of stress, it looked healthy and it had been routinely maintained.

The region is in a fourth year of severe drought, but Pasadena was soaked with record rainfall in July. It’s not yet clear if either played a role.

“Healthy trees can fail. That’s oftentimes what we hear people say: ‘Well, the tree was really green and it looked good and it fell over,’” said Larry Costello, horticulture advisor emeritus for the University of California Cooperative Extension.

“They may be healthy, but not structurally strong. The root system may have some defect that compromises it, the structural stability or the branches.”

The area was hit by a strong storm about two weeks ago. Costello said the tree may have been damaged if there were strong winds.

“It could have had internal cracks and splits that are hard to detect,” he said.

Eight children between ages 6 and 8 were injured as they tried to scramble to safety when the tree fell Tuesday afternoon. Six had only minor injuries.

One young boy remained in critical condition, while one young girl’s condition was upgraded from critical to guarded at the pediatric trauma unit at the Los Angeles County University of Southern California Medical Center, Pasadena Fire Department spokeswoman Lisa Derderian said.

The towering pine was located in Pasadena’s Brookside Park, just outside the grounds of the Kidspace Children’s Museum.

The other campers returned Wednesday to the museum’s weeklong Water Wonders Camp, with counselors available to talk if they were needed.

Derderian said it was otherwise business as usual at the always-buzzing park that’s adjacent to the Rose Bowl, other sports fields and a major swimming-and-diving complex.

The museum offers weekly day camps during the summer for children ages 5 to 9.