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The US has “several” intelligence indications that Iran has put portions of its air defense system on “high alert” in recent days, following unexplained explosions at key facilities tied to the country’s military and nuclear programs, according to a US official who is closely tracking developments.

The change in alert status means Iranian surface-to-air missile batteries would be ready to fire at targets perceived to be a threat.

The official would not say how the US picked up on these indicators, but American satellites, spy planes, and ships routinely operate in nearby international airspace and waters where they continuously monitor Iranian activity.

Several US military officials declined to publicly comment on whether the US has intelligence related to Iran’s alert status.

The US currently assesses the Iranian alert is not part of a training exercise but is a response to recent events and nervousness over whether there is an unknown threat to the regime in the wake of multiple mysterious explosions at various facilities this month.

A struggle to explain

Iran has struggled to explain the cause of those incidents, including a fire that caused major damage to a site that has been key to the country’s uranium enrichment program, prompting questions about potential sabotage.

International speculation has centered on a theory that Israel may be behind some of the explosions, even though US officials originally said the Israelis had assured them they were not responsible.

However, Israeli Defense Minister Benny Gantz did not rule that possibility out while discussing the issue on July 5, saying, “Not every incident that transpires in Iran necessarily has something to do with us. … All those systems are complex, they have very high safety constraints and I’m not sure they always know how to maintain them.”

Publicly, the US has not commented on a potential Israeli connection. Top US officials are trying to learn more about the explosions and who, or what, might have been responsible, the official said.

One of the most critical incidents came July 2 when a fire caused significant damage to a building at Iran’s Natanz nuclear plant. That site was previously the target of a cyberattack widely believed to have been carried out by Israel and the US that came to light with the 2010 discovery of the Stuxnet computer virus.

Iranian state TV has previously cited an anonymous security official saying an investigation of the fire at the Natanz nuclear complex found “no evidence” of sabotage.

However, the BBC’s Persian service has also reported receiving an oddly worded statement from an unknown group calling themselves “Cheetahs of the Homeland,” who claimed they were behind that explosion without providing evidence.

Other unexplained incidents that have occurred in recent weeks include a large blast near the town of Parchin and its military complex. Another explosion hit the Zargan power plant in Ahvaz. That incident was quickly followed by a suspected chlorine leak that made dozens ill in southeast Iran.

While the cause of these incidents still remains unclear, top US military officials are also beginning to indicate privately that it appears increasingly unlikely all these events are due to industrial accidents given the number that have occurred.

Gen. Kenneth McKenzie, commander of US Central Command, has only provided vague answers when asked about the series of incidents.

“I’m going to leave that one alone. The Iranians — they’re talking a lot about it. I just listen to what the Iranians say on that,” he told reporters earlier this month.

ut just days after making those comments, McKenzie appeared to suggest there was intelligence on what might have happened.

“We have seen and observed those explosions in Iran. I’m not going to be able to speculate what that may or may not have done to the Iranian nuclear program,” he said.

One key concern is that Iran could “lash out” and counterattack in an unpredictable fashion if it believes it may be under attack by Israel or the US, the official said.

The US is also concerned that Iran’s unreliability in operating its air defense systems means moving to a high alert status could also pose a threat on its own.

In January, a civilian Ukraine airliner was shot down shortly after takeoff by a surface to air missile launched mistakenly by Iran.

Meanwhile, Iran’s military tactic of putting its air defenses on alert may not really address the potential threat it perceives, the official said.

There is no indication that any fighters, bombers or missiles were launched against Iran, meaning any potential attack could have been been ground- or possibly cyber-based.

But if there are opposition groups on the ground in Iran conducting attacks against key facilities there, it is not clear to the US if any outside personnel, money or organization is supporting such an effort.

CNN’s Zachary Cohen contributed to this report