By Tim Lister, with reporting by Kathleen Johnston and Pam Benson



The Sentinel drone that crashed in Iran last week was on a surveillance mission of suspected nuclear sites in the country, U.S. military officials tell CNN.

Previously, U.S. and NATO officials had said the drone was on a mission to patrol the Afghan-Iran border and had veered off course.

The officials say the Afghan government was unaware of the use of its territory to fly surveillance drones over Iran, and that the CIA had not informed the Defense Department of the drone's mission when reports first emerged that it had crashed. One official told CNN that the U.S. military "did not have a good understanding of what was going on because it was a CIA mission."

In Kabul Wednesday U.S. Defense Secretary Leon Panetta refused to comment directly on the specifics of the drone's mission but did not deny that it had been spying on Iran and said the drone program carried out "important intelligence operations which we will continue to pursue."

The RQ-170 Sentinel is one of the United States' most sophisticated drones and flies at up to 50,000 feet. It is designed to evade sophisticated air defenses. One former intelligence official told CNN that it's "impossible to see" and discounted Iranian claims that it had been brought down by some form of electronic counter-measures.

"It simply fell into their laps," he said - after satellite communication was lost.

An Iranian engineer working on the captured drone claimed to the Christian Science Monitor that Iran was able to overtake the drone control system and land it. The engineer described to reporter Scott Peterson how Iran was able to exploit a vulnerability in the drone system.

House Intelligence Committee Chairman Mike Rogers said categorically that Iran had nothing to do with the fate of the aircraft.

"I will say without hesitation that this was not something that anyone had anything to do with, coming down with, other than a technical problem," he said. "I will tell you there was a technical problem that was our problem, nobody else's problem. I think there is a lot of PR going on."

But Panetta did not categorically rule out the possibility when asked whether the drone could have been taken down by a cyberattack or jamming.

"You can make all kinds of guesses at this point. Obviously there's nothing that you can rule out and nothing that you can rule in right now," Panetta told Fox News earlier this week.

The former official doubted the intelligence value of the drone to the Iranians. For example, the sensors on the underbelly of the drone would have been badly damaged, he said. And coolant could have damaged the main computer. The sensors are designed to detect airborne chemical traces that might come from a nuclear facility or intercept cell-phone conversations.

But Bill Sweetman, the editor in chief of Defense Technology International, believed everything inside the plane would be in one piece based on what he could see from the video released by the Iranians. Sweetman added this aircraft did not represent the crown jewels of U.S. technology. "This is not the most advanced stealth technology out there."

Iranian officials have said the drone came down over eastern Iran, hundreds of miles from the cluster of nuclear sites in central and north-western Iran. Some said it came down close to the city of Kashmar in Khorasan province; others that it was recovered in the Tabas desert to the south-west.

The Iranian ambassador to the United Nations, Mohammad Khazaei, told CNN, "It was brought down somewhere in about some 150 miles or 250 kilometers deep into the Iranian territory in the province of Khorasan in the northern city of Tabas."

The Tabas desert is a remote region used by the U.S. military in 1980 as a staging area during the abortive attempt to rescue the hostages at the U.S. embassy in Tehran.

According to experts who follow Iran's nuclear program, this part of the country is not known for nuclear facilities. More than a decade ago there were reports that Iran had developed an enrichment facility with North Korean help in the Tabas Desert. But its existence was never confirmed, and an official at the International Atomic Energy Agency said its delegations had not been to the area.

David Albright, the president of the Washington-based Institute for Science and International Security (ISIS), expressed doubts that North Korea would have shared centrifuge technology with Iran at a time it was developing its own enrichment facilities.

The area is known for its uranium mines. which Iran is under no obligation to subject to international inspection. The main mine is at Saghand in Yazd province, to the south-west of the Tabas desert. According to ISIS, it supplies uranium ore concentrate to a facility at Ardakan. Other mines in Iran's eastern provinces are being developed, according to the IAEA.

Equally the drone may have been traveling across eastern Iran toward other known or suspected nuclear sites, experts say. In its most recent report, the IAEA identified many of these in and around Tehran as well as around Qom and Isfahan.

It is still unclear where the drone was based in Afghanistan. Previously reported sightings of the RQ-170 Sentinel were at the Kandahar base in southern Afghanistan. But Afghan officials say this mission began at the former Soviet air base at Shindand in western Herat province. A straight line from there to known nuclear facilities identified by the IAEA in north and central Iran would have taken the drone close to Kashmar.

The International Security Assistance Force in Afghanistan has refused to confirm the Sentinel left Shindand.

CNN's Kathleen Johnston and Pam Benson contributed to this report.