The 75-foot-tall, 75-year-old tree cracked and fell around 4:45 p.m. Tuesday just as 33 kids were let out of Water Wonder Camp, a summer day camp at the Kidspace Children's Museum.
Several children were outside waiting to be picked up by their parents when the giant tree hurtled toward them.
PHOTOS: Kids rescued after tree falls near Pasadena children's museum
Museum staff, police, firefighters and parents rushed to the scene. The fire department dispatched its urban search-and-rescue team to saw off branches to free trapped kids.
In all, eight children were injured. Six suffered only minor injuries, but two children were transported to Los Angeles County-USC Medical Center in critical condition. They suffered blunt-force trauma injuries but were expected to be OK.
Children who were near the tree when it well fell said the sound of the tree crashing down was like thunder.
"I thought it was rain coming, but it was actually the tree," said Subin Park, a camper.
Keelie Pellman returned to the camp on Wednesday a bit shaken.
"It was really frightening and scary. I was very grateful that I wasn't under that tree," Keelie said.
Her grandmother Barbara Palmer said she was apprehensive about getting near other large trees.
"But I explained that that was probably a very unusual occurrence," Palmer said.
The cause of the fall remains unclear. The grass surrounding the tree was green, and the day only saw minimal wind. Officials said crews conduct regular maintenance and care of all the city's trees.
The museum's CEO Michael Shanklin said he is gathering incident reports from all of the staff members involved in the incident to piece together what happened.
Prior to the independent investigation, the city's certified arborist conducted a preliminary assessment of the tree and could not find conclusive reasons for the fall, according to a press release from the city.
Lisa Smith, an arborist who specializes in assessing the risk of falling for trees, said an investigation could take several weeks.
"Oaks, pines and eucalyptus are evergreen, and they are at their heaviest in late-summer, early fall, so they are the most prone to failing," Smith said.
Although California is in the middle of a severe drought that has prompted residents and municipalities alike to drastically cut back on water use, it wasn't known if that could have played any role.
The huge pine was located in Pasadena's Brookside Park, just outside museum grounds near the Rose Bowl. City officials said the area around the site will remain closed to the public until further notice.
The museum, which opened Wednesday for normal summer camp hours, offers weekly day camps during the summer for children ages 5 to 9.
This week's Water Wonders Camp will be followed next week by Outer Space Camp, which is to include visits from space experts at the nearby Jet Propulsion Laboratory.
The Associated Press contributed to this report.