A Falcon 9 rocket set to take a satellite into space on Labor Day weekend exploded during testing days before. Courtesy of http://www.uslaunchreport.com/. (USLaunchReport)

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, Fla., 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 launchpad 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 leased by 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 launchpad 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.

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 Sept. 1. The rocket blew up while it was being fueled ahead of an engine test fire, creating a huge fireball that charred the launchpad 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.”

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.

At a conference in Mexico this week, Musk said that finding out what went wrong is the company’s “absolute top priority,” but he said what caused the explosion is still unknown.

“We’ve eliminated all of the obvious possibilities for what occurred there,” he said. “So what remains are the less probable answers.”

He didn’t say what those might be.

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

A SpaceX statement said that the “Accident Investigation Team has an obligation to consider all possible causes of the anomaly, and we aren’t commenting on any specific potential cause until the investigation is complete.”

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.

This week, 10 Republican House members, many friendly to ULA, told NASA that SpaceX should not be leading the investigation and that authority should be turned over to the federal government.

Even though the investigation continues, SpaceX has said it intends to return to flight as soon as November, a timeline that has drawn industry skepticism.