While Israel strutted after announcing to the world on Wednesday that it had destroyed Syria’s nascent al-Kibar nuclear reactor in 2007, a US-based think tank published a report the same day suggesting that there is another atomic facility that Jerusalem should be worrying about.

The paper, released by the Institute for Science and International Security, revisits a claim made in 2015 by the German daily Der Spiegel that Syrian dictator Bashar Assad was constructing another nuclear reactor, this one underground, near the city of Qusayr, along the Lebanese border.

The findings of the new report are inconclusive, but determine that the Syrian regime certainly built something in the ground under Qusayr and that some of the claims made by Der Spiegel are supported by publicly available evidence, namely satellite images and geological data.

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Even if the underground site is not home to a reactor, it may be used to store leftover nuclear material from al-Kibar or is in some capacity connected to Syria’s chemical weapons or ballistic missile programs, according to the report.

There is not much to see of the site from satellite images: just five gray rectangular buildings in a valley between two hills and, further down the road, a guard shack.

“We’re always wondering if this is going to be the next site that Israel is going to strike,” David Albright, one of the paper’s authors and president of the institute, told The Times of Israel over the phone, shortly after its publication.

The Israeli military refused to comment on the report.

It’s partially a coincidence that the report came out on the same day Israel formally acknowledged that it bombed the Syrian reactor in Deir Ezzor in 2007, Albright said.

The Washington, DC-based think tank, which focuses on nuclear non-proliferation and related issues, started checking the claims made in Der Spiegel shortly after the report came out in 2015, but nothing much came of it. The authors returned to the topic a few months ago and were still working on it when the news broke early Wednesday morning.

By publishing now, ISIS (the think tank was founded well before the terrorist group) hoped to garner interest in the Qusayr facility and potentially get some of its remaining questions answered, Albright said.

According to the institute, the construction of the site may have been effected with assistance from North Korea, which “is known to export its mining and excavation expertise to countries such as Syria,” as it did in the construction of the al-Kibar facility in the early 2000s.

One of the lingering concerns following Israel’s 2007 strike was that while the reactor may have been destroyed, many of the materials and pieces of equipment necessary for manufacturing a nuclear weapon remained unaccounted for, including “stockpiles of natural uranium, fuel fabrication capabilities, and even possibly plutonium separation capabilities,” according to the report.

Citing intelligence officials, Der Spiegel reported in 2015 that those remaining nuclear materials were being used to construct a subterranean nuclear reactor in Qusayr.

Wednesday’s report notes that building an underground reactor would be an exceedingly difficult feat, with significant technical challenges associated with it, but that it is “not impossible.”

Satellite images show that large amounts of limestone were excavated from the site and that efforts were made to conceal that fact.

The report also finds the site’s close proximity to an underground aquifer could provide the water necessary to cool a nuclear reactor, though that is not how such cooling is generally achieved. A satellite photo from 2012 showed a mobile drill rig, which would have been used to tap into the groundwater.

Using the existing aquifer would mean Syria could forgo aboveground cooling facilities.

“Such a strategy, while unusual, would be consistent with Syrian efforts to suppress observable signatures, such as Syria did at the al-Kibar reactor,” the authors wrote.

They also note that using groundwater to cool a nuclear reactor is not unprecedented, with an American facility in Alaska using such a setup.

“We believe that this site warrants inspection by the International Atomic Energy Agency,” wrote the report’s authors, who recognized that such an inspection might not be feasible until the end of the Syrian civil war.