Although unconfirmed, officials said the families were headed to Montana for a ski trip.
Butte resident Martha Guidoni and her husband raced to the scene when they witnessed the plane spiral down.
"It looked like a meteorite hit the ground. ... I've never seen anything so scary in my life," she said.
During a news conference Sunday evening, NTSB officials said they had few details about the crash.
"We are just beginning our investigation," board investigator Kristi Dunks said. "We don't have a lot of information at this time. ... Certain family members were contacted. ... At this point, I don't have an exact number."
The single-engine plane was manufactured in 2001 and owned by Eagle Cap Leasing Inc., based in Enterprise, Ore.
It remains unclear whether there was a second pilot onboard. According to Nance, this plane can be flown by only one pilot, and the identity of that person is unknown so far. There is some speculation that it was the owner of the aircraft.
However, because of a lack of flight recorders on board, early details are expected to remain scanty.
"There are a lot of questions, and there are answers that we don't have," Nance said.
Weight and balance seem to be key factors so far, but further investigation could uncover more things in play.
Half of the passengers who died were children. Among the passengers was a family of five parents and three toddler-age children.
Tom Hagler, owner of Table Mountain Aviation in Oroville, greeted the group in California.
"They were young, about 6 to 10 years old, and at least a couple of adults and the pilot," he said.
The last fatal plane crash that occured was in February, when a Continental Airlines plane fell on a house in a suburb of Buffalo, N.Y., killing all 49 passengers onboard.
Across the world, another plane crash killed two people.
A FedEx cargo plane crashed in Tokyo, killing an American pilot and co-pilot.