As tensions rise Iran’s bloody crackdown on protests across the countrysatellite images show activity on two Iranian nuclear sites bombed last year by Israel and the United States, which may be a sign that Tehran is trying to cover up efforts to salvage the material left there.

Images from PBC’s Planet Labs show that roofs have been rebuilt on two damaged buildings at the Isfahan and Natanz facilities, the first major activity seen by satellite at one of the country’s stricken nuclear facilities since Israel’s 12-day war with Iran in June.

Rooftops seen in Isfahan and Natanz

Those blankets block satellites from seeing what’s happening on the ground—right now, that’s the only way inspectors of the International Atomic Energy Agency to monitor the site, because Iran blocked access.

The new roofs do not appear to be a sign of the reconstruction that has begun on the badly damaged structures, said experts who inspected the sites. Instead, they are likely part of Iran’s efforts “to assess whether key assets — such as limited stockpiles of highly enriched uranium — survived the strikes,” said Andrea Stricker, who studies Iran for the Tehran-sanctioned Washington-based Foundation for Defense of Democracies.

“They want to be able to get to whatever returned property they can get to without Israel or the United States seeing what’s left,” she said.

The main above-ground enrichment building at Natanz was known as the Pilot Fuel Enrichment Plant. Israel hit the building on June 13, leaving it “functionally destroyed” and “severely damaging” the underground halls that house the cascades of centrifuges, IAEA Director General Rafael Mariano Grossi said at the time.

A subsequent US attack on June 22 hit the Natanz underground facilities with bunker busting bombs. Grossi said “Face the Nation with Margaret Brennan” that intelligence from Iran indicated that the ensuing US strikes caused “severe damage” but not “total damage”.

“Iran has the capacity there; the industrial and technological capacity. So if they want to, they can start doing this again,” Grossi said.

PBC images from Planet Labs show that Iran began building a roof over the damaged power plant in December. By the end of the month, they finished the work on the roof. Iran has not given any public recognition for that work. Natanz’s electrical system appears to still be destroyed.

In Isfahan, Iran began building a similar roof over a structure near the northeast corner of the facility, completing work in early January. The building’s exact function is not publicly known, although the Israeli military said at the time that its strikes on Isfahan targeted sites there associated with the production of centrifuges. The Israeli military did not respond to requests for comment on the construction.

Meanwhile, pictures show two tunnels in the mountain near the Isfahan facility are filled with earth, a measure against missile attacks that Iran also carried out just before the June war. The third tunnel appears to have been cleared of dirt, with a new set of walls built near the entrance as an obvious security measure.

Sarah Burkhard, a senior fellow at the Washington-based Institute for Science and International Security, who has long observed Iran’s nuclear sites, said the roofs appear to be part of an operation “to bring back any remaining assets or debris without letting us know what they’re taking out of there.”

Sean O’Connor, an expert at the open-source intelligence firm Janes, agreed that the goal was probably to “obfuscate the activity rather than, say, repair or rebuild the structure for use.”

Iran has not publicly discussed activities at the two sites. The IAEA, the United Nations’ watchdog agency, did not respond to requests for comment.

US President Trump he has repeatedly demanded that Iran negotiate a deal for its nuclear program to prevent threats of US military strikes to crush protesters in the country. USA moved USS Abraham Lincoln and several guided-missile destroyers to the Middle East, but it remains unclear whether Mr. Trump they will decide to use force.

Mr Trump said earlier this month that an “armada” of warships was headed for Iran “just in case”.

Lincoln was in the Pacific when the president first issued a warning on social media that the US was “ready” to attack Iran if the regime killed anti-government protesters.

Since the protests began, Human Rights Defenders in Iran, a US-based non-governmental organization, estimates that more than 4,000 protesters killed so far and says that number is likely to rise. Two sources, including one inside Iran, told CBS News earlier this month that at least 12,000 and possibly as many as 20,000 people were killed.