KABC/CNN; Twitter screenshot by CNET

SpaceShipTwo, a suborbital space plane designed for space tourism and operated by Virgin Galactic, crashed Friday during a test flight in California's Mojave Desert.

California Highway Patrol first confirmed one fatality, according to local NBC TV affiliate KGET, which also reported that one person with major injuries has been transported by emergency helicopter to a nearby hospital. Virgin Galactic confirmed the status of the pilots in a press conference today, adding that the two pilots on board the aircraft were trained and employed by Scaled Composites, the company's aerospace partner. The identities of the pilots remain unknown.

According to a tweet from the Associated Press, a witness said the plane exploded in midair after its rockets ignited. That ignition is a key stage of SpaceShipTwo's self-flight process, where it detaches from the larger WhiteKnightTwo aircraft that ferries it to cruise altitude. WhiteKnightTwo landed safely, Virgin confirmed on Twitter earlier today.

It's unknown what caused the accident. Officials with the National Transportation Safety Board are expected to begin an investigation on Saturday into the cause of the accident, which could take several days.

"Space is hard and today was a tough day," said George Whitesides, Virgin Galactic's CEO, in the company's press conference this afternoon. "But we believe we owe it to the folks who were flying these vehicles as well as the folks who have been working so hard on them to understand this and to move forward."

Virgin Galactic is the space tourism company owned by billionaire Richard Branson's Virgin Group. Its goal is to operate a fleet of space planes, like SpaceShipTwo and WhiteKnightTwo, that will carry passengers into suborbital space for as much a $250,000 a person.

SpaceShipTwo was unveiled in 2009 and completed its first self-powered flight in 2013. SpaceShipTwo was originally developed as part of a joint venture between Virgin Galactic and Scaled Composites. Virgin became the sole owner of that venture, called The Spaceship Company, in 2012.

SpaceShipTwo, designed to carry two pilots and six passengers, has completed 54 test flights in the last five years, the latest being a controlled "glide flight" last August to test stability during descent. The last powered flight of SpaceShipTwo occurred in January of this year, as Virgin Galactic was changing the aircraft's rubber-based solid fuel that caused engine instabilities to a new, better-performing thermoplastic-based solid fuel developed by Scaled Composites. Today's flight was the first to use the new fuel while the aircraft was in the air.

Branson has said this year that he hopes to be on the first commercial flight either later this year or sometime in early 2015. More than 700 people have booked a flight with Virgin Galactic, including high-profile names like Leonardo DiCaprio and Stephen Hawking. It's unclear how the SpaceShipTwo crash will affect the company's commercial operations.

Today's SpaceShipTwo crash follows another space-launch accident earlier this week. On Tuesday, Orbital Sciences Corp.'s Antares rocket lifted off from NASA's Wallops Island Flight Facility carrying an unmanned Cygnus spacecraft with 5,000 pounds of supplies bound for the International Space Station.

An unknown error caused a first-stage engine malfunction and the rocket ignited in flames in midair, sending the vehicle back to Earth, where an Orbital official deliberately engaged the flight termination sequence to combust the rocket above, and not on, the launch pad to avoid further damaging the area. No injuries were reported in the incident.

Virgin Galactic intends to hold a second press conference on Saturday.

Update, 2:30 p.m. PT: Adds details from Virgin Galactic's 2 p.m. PT press conference.