The long-running feud between Elon Musk’s space company and its fierce competitor United Launch Alliance took a bizarre twist this month when a SpaceX employee visited its facilities at Cape Canaveral, Florida, and asked for access to the roof of one of ULA’s buildings.

About two weeks earlier, one of SpaceX’s rockets blew up on a launch pad while it was awaiting an engine test. As part of the investigation, SpaceX officials had come across something suspicious they wanted to check out, according to three industry officials with knowledge of the episode. SpaceX had still images from video that appeared to show an odd shadow, then a white spot on the roof of a nearby building belonging to ULA, a joint venture between Lockheed Martin and Boeing.

The SpaceX representative explained to the ULA officials on site that it was trying to run down all possible leads in what was a cordial, not accusatory, encounter, according to the industry sources, who spoke on the condition of anonymity because of the ongoing investigation.

The building, which had been used to refurbish rocket motors known as the SMARF, is just more than a mile away from the launch pad and has a clear line of sight to it. A representative from ULA ultimately denied the SpaceX employee access to the roof and instead called Air Force investigators, who inspected the roof and didn’t find anything connecting it to the rocket explosion, the officials said.

Fireball

The interaction between SpaceX and ULA has not been previously reported. It is the latest odd development in the mystery surrounding the explosion of SpaceX Falcon 9 rocket on September 1. The rocket blew up while it was being fuelled ahead of an engine test fire, creating a huge fireball that charred the launch pad and rattled buildings miles away.

Elon Musk, SpaceX’s founder and chief executive, has called the failure “the most difficult and complex” the company has ever had. About a week after the explosion, he pleaded with the public to turn in video or audio recordings of the blast and said that the company has not ruled out sabotage as a factor.

“Particularly trying to understand the quieter bang sound a few seconds before the fireball goes off,” he wrote on Twitter. “May come from rocket or something else.”

Breach

Since then, SpaceX, which is leading the investigation with help from the Air Force, Nasa and the Federal Aviation Administration, said it is narrowing down on the cause of the explosion, focusing on a breach in a second-stage helium system.

The Air Force’s 45th Space Wing, which is helping SpaceX with the investigation, declined to comment because the investigation is ongoing.

SpaceX and ULA are heated rivals that are competing over national security contracts that together are worth hundreds of millions of dollars. For nearly a decade, ULA had a monopoly on those contracts as the only launch provider certified by the Air Force.

But in 2014, SpaceX sued the Air Force for the right to compete. Last year, the parties settled and SpaceX was finally granted its certification. As a result, ULA fired its chief executive and hired a new one who vowed to compete with SpaceX.