LOS ANGELES, California (CNN) -- A third person died Friday from an explosion at a rocket test pad operated by a private company in California's southern Mojave Desert, according to the nursing supervisor at Kern Medical Center in Bakersfield.
A bird's eye view of the scene in Mojave, California, shows charred wreckage and large pieces of debris.
The explosion, which occurred Thursday at the Mojave Air and Space Port in Mojave, California, initially killed two people and critically injured four others.
On Friday three people remained hospitalized -- two in critical condition, one in serious condition.
The six were employed by Scaled Composites, an aerospace development company founded 25 years ago by Burt Rutan, the aerospace engineer who designed the first plane to fly nonstop around the world without refueling.
According to Rutan, the blast occurred during a test involving nitrous oxide flowing through fuel injectors. A source said the workers were testing components of a new rocket motor for SpaceShipTwo, which is a spaceship being built for Virgin Galactic, Richard Branson's space tourism company.
"We were doing a test that we believed was completely safe. ... We don't know why it exploded," said Rutan, who was not at the site at the time of the accident. Watch the aftermath of the explosion »
The explosion happened during a "cold fire test," he said.
A spokeswoman for the space port, which is about 80 miles north of Los Angeles, said the blast was on a remote pad and that the area had been contained.
Aerial video of the blast aftermath showed a charred and twisted flatbed trailer attached to a truck cab with a large silver tank next to it. Large pieces of debris appeared to be strewn for hundreds of yards from the center.
Rutan won the Ansari X-Prize in 2004 for the first repeat sub-orbital space flight with his pioneering SpaceShipOne.
SpaceShipTwo is a small carbon fiber craft designed to carry seven passengers up to 65 miles above the earth, traveling at a speed of about 12,000 mph. A ticket for such a ride was expected to cost $200,000.
Entering the edge of space those on board will experience weightlessness for about 15 minutes before beginning the journey back to earth.
"What we're trying to do is make space travel affordable and fun. Two years from now we'll be starting to take ordinary people into space," Branson told CNN in 2006. E-mail to a friend
CNN's Kate Tobin and Mike Ahlers contributed to this report.