NASA failed to follow its general policies and opened the door to “questions about inherent conflicts of interest” by allowing Space Exploration Technologies Corp. to lead the main investigation into a failed 2015 rocket launch by the company, according to the agency’s inspector general.

A report issued Tuesday by Paul Martin, the National Aeronautics and Space Administration’s internal watchdog, faults agency officials for permitting SpaceX, as the company is called, to run the central probe into the explosion of its Falcon 9 rocket two minutes after liftoff in June 2015.

The accident destroyed the unmanned booster and a capsule carrying $118 million of cargo destined for the international space station, producing delays and complications for NASA’s operation of the orbiting laboratory. Federal and company investigators eventually agreed the accident was caused by a chain of events stemming from the sudden failure of a relatively basic structural support, called a strut, inside the rocket’s upper stage.

On the first anniversary of the accident, the report described what it called NASA’s deviation from “existing agency risk classification” procedures for launches. The inspector general concluded that because NASA officials have treated all commercial-cargo flights as carrying low-priority materials, they could justify using high-risk boosters and enforcing minimal launch constraints.

Moreover, the report found the agency relies heavily on SpaceX and Orbital ATK Inc., the other company that has flown NASA cargo into orbit, “to assess and mitigate risk for launches.” But the lack of uniform assessments, according to the inspector general, frequently results in confusion and changes in what is acceptable. The report, for example, recounted that NASA officials historically expected a one-in-six overall failure rate for cargo deliveries. Yet before an unsuccessful launch of an Orbital ATK Antares rocket in 2014, one NASA review team projected the likelihood of failure as “50/50,” the report noted.