A U.S. House panel is asking NASA to explain why it hasn't launched an independent inquiry into a June explosion involving a SpaceX Falcon 9 rocket — after the agency promptly did so for a similar incident involving the Antares rocket from Orbital ATK Inc in October 2014.

Both of the launches were paid for by NASA, with separate contracts costing over $1 billion each to bring cargo to the International Space Station. Both SpaceX and Orbital are investigating their respective explosions, with oversight from the Federal Aviation Administration. The accidents have however raised concerns about the country's growing reliance on commercial launch contracts and investigative lapses on the incidents.

Lamar Smith, chairman for the U.S. House Committee on Science, Space and Technology, wrote to NASA Administrator Charles Bolden, saying the discrepancy in the agency's treatment of the accidents not only raises concerns about fairness and equity but also questions the fidelity of the investigations. Smith adds that, given the amount of each contract ($1.6 billion for SpaceX and $1.8 billion for Orbital), NASA definitely had reason to look into the accidents.

When Orbital's Antares rocket crashed, NASA formed an independent review team to investigate the incident and aimed to learn from any lessons that could be gleaned from the explosion. When SpaceX's Falcon 9 rocket failed, NASA only requested that an agency representative be included in the probe.

This response was "perplexing" to Smith.

The letter arrived a week after 14 lawmakers informed NASA and the U.S. Air Force that they have serious concerns over SpaceX leading its own investigation into its own rocket failure. It should, however, be noted that many of these lawmakers come from districts representing United Launch Alliance, SpaceX's competitor for launch contracts.

SpaceX has not commented on the issue, but the Air Force has spoken up, saying it was involved in and is closely following the investigation being carried out by SpaceX. NASA also said it is participating in all investigations, adding that it's "confident both companies will understand the specifics of their respective mishaps, learn from them, and correct the issues so they can return to flight."

According to sources with knowledge on the issue, Orbital has already submitted an investigation report about the accident in October to the FAA.