There are many mysteries involved in the crash of a U.S. surveillance drone in Iran. Among them: What could have brought it down?

Defense officials said Monday they had no indication that it was shot down. Iran’s semiofficial Fars New Agency, meanwhile, has claimed the drone was “downed with help from the Iranian military’s electronic warfare unit.”

But experts say that explanation strains credulity.

Such an attack would be extremely difficult to execute, especially with an aircraft such as the RQ-170, the model believed to have gone down in eastern Iran. Although the Air Force acknowledged the existence of the drone in 2009, very little is known about its technical attributes. Its operational use has not been described publicly.

“It’s pretty tough to get into them in cyber and even tougher to tell them, ‘Go crash,’ ” said one expert who was not authorized to speak for the record about sensitive systems.

Experts said the primary communications antennas on the RQ-170 are on top of the aircraft, which makes it less susceptible to being hacked. If hackers wanted to intercept or modify the signal, they would have had to have been near the signal’s “footprint.”

That seems unlikely, experts said, given that the RQ-170 has a special coating designed to help it avoid detection by enemy radar.

And even if all of those obstacles could be overcome, experts said, it’s unlikely that they would be overcome by Iran.

The Islamic Republic does have a cyberwar unit. But that unit’s capabilities are believed to lag far behind those of countries including the United States, Russia, Israel and China.

”If this happened, it is a 95 percent chance that it just malfunctioned,” one senior Pentagon official said. “There are a lot of things that can fail.”