WASHINGTON — Space Exploration Technologies Corp. (SpaceX) has discovered the root cause of a premature engine shutdown during the company’s first paid cargo flight to the international space station in October, but the Hawthorne, Calif., rocket and spacecraft maker is not ready to make the results of its months-long investigation public, a company executive said.

“We’re doing one of the final out briefs on the most probable cause for the engine issue with [NASA’s international space station program manager, Michael Suffredini] later this week,” SpaceX President Gwynne Shotwell said Dec. 11 at a Washington Space Business Roundtable luncheon here. “We’re not going to release what we found but I think we’ve got a good most probable cause identified. The data supports that.”

SpaceX spokeswoman Katherine Nelson said Dec. 12 that Shotwell “was merely stating that she was not going to release any information at yesterday’s Washington Space Business Roundtable luncheon.”

“SpaceX has conducted the investigation with the full cooperation of NASA, and results will be made public only after our customers have been fully briefed and everyone agrees with the findings,” Nelson said.

One of the nine first-stage engines on SpaceX’s Falcon 9 rocket shut down prematurely 79 seconds after liftoff Oct. 7 at the start of an otherwise successful space station cargo run. The only publicly released details about the engine failure so far come from a statement SpaceX put out the day after the launch, which boosted the company’s Dragon cargo capsule toward the station.

“We know the engine did not explode because we continued to receive data from it,” SpaceX said in an Oct. 8 press release. “Our review indicates that the fairing that protects the engine from aerodynamic loads ruptured due to the engine pressure release, and that none of Falcon 9’s other eight engines were impacted by this event.”

Josh Byerly, a NASA spokesman at the Johnson Space Center in Houston where the space station program office is located, did not reply to a Dec. 11 request for comment about SpaceX’s investigation.

NASA is SpaceX’s single largest customer by revenue, but the company is adding more U.S. government business to a manifest that, on a per-launch basis, is primarily commercial, Shotwell said.

Shotwell said Dec. 11 that while about 60 percent of the launches in SpaceX’s $4 billion backlog are commercial, a full 40 percent are now U.S. government launches. Commercial launches accounted for about 70 percent of the company’s manifest earlier this year, she said.

The shift is due to SpaceX’s winning business from the U.S. military, which in late November decided it would allow several firms to compete with incumbent launch services provider United Launch Alliance (ULA) of Denver for as many as 14 of the 50 launches the Defense Department plans to buy over the next five years. ULA is a joint venture of Boeing and Lockheed Martin.

On the commercial side of its business, Shotwell said SpaceX will debut an upgraded Falcon 9 rocket, which features the new Merlin 1D engine, sometime in 2013. In that launch, which will boost the Canadian Space Agency’s Cascade Smallsat and Ionospheric Polar Explorer satellite to orbit, SpaceX will also debut a 5.2-meter payload fairing. Falcon 9 needs the larger fairing to accommodate the big communications satellites SpaceX is under contract to launch for commercial fleet operators AsiaSat and SES, among others.

SpaceX has at least seven launches on the manifest for 2013, including the debut of the Falcon Heavy from the company’s new West Coast launch pad at Vandenberg Air Force Base, Calif.

The current Falcon 9 features a 3.6-meter fairing, uses nine Merlin 1C liquid oxygen and kerosene-fueled engines, which are less powerful than the Merlin 1D engines SpaceX has been testing at its McGregor, Texas, rocket facility. The first NASA mission in which a Falcon 9 with Merlin 1D engines will be used is a space station cargo run now scheduled for July, according to an internal NASA manifest.

Before that mission, which would be SpaceX’s third for NASA under a $1.6 billion Commercial Resupply Services contract awarded in 2008, the company plans to launch three commercial payloads with the upgraded Falcon 9. However, Shotwell said, there is no contractual requirement for SpaceX to test the upgraded Falcon 9 before using it to resupply the space station.

Meanwhile, SpaceX has made no firm decision on a dedicated commercial launch site, Shotwell said. The company is considering building in Texas, Florida and even Puerto Rico, although in Puerto Rico “there are some political difficulties” to consider, Shotwell told SpaceNews.