The cancellation of next month’s U.S.-North Korea summit has delayed a confrontation over key elements of Pyongyang’s advanced weapons programs, including what some Western intelligence officials say is a second uranium-enrichment site operating inside the country.

Trump administration officials, in addition to seeking a deal to halt North Korea’s nuclear advances, were hoping the June 12 summit would lead to a new openness by leader Kim Jong Un about a number of clandestine activities, including the stockpiling of chemical nerve agents and the sale of nuclear and missile technology to Syria and perhaps other clients, according to current and former U.S. officials familiar with preparations for the talks.

The chance for increased transparency was seen as one of the biggest potential benefits from the diplomatic thaw with North Korea, as it could allow for greater confidence in assessing the communist regime’s capabilities and intentions, as well as uncovering its dealings with other governments seeking weapons of mass destruction. Until now, North Korea’s nuclear and missile programs have been shrouded in nearly impenetrable secrecy, explaining U.S. analysts’ shocked reactions to Pyongyang’s leaps in long-range missile technology over the past two years.

Among the more intriguing recent mysteries is the suspected enrichment facility, a subject of discussion among U.S., European and East Asian intelligence agencies since it was first described by a North Korean defector a few years ago, according to current and former U.S. and European officials briefed on the matter. Such a facility, if confirmed to exist, could potentially provide North Korea with a significant undocumented source of enriched uranium for nuclear bombs.

Additional details about the suspected site were described in a computer slide presentation prepared this month by the Institute for Science and International Security, a Washington research organization that has published multiple studies detailing North Korea’s nuclear weapons program. North Korea is known to operate one uranium-enrichment plant, at Yongbyon, 60 miles north of Pyongyang. It allowed visiting U.S. officials to see that facility in 2010 and has declared since then that no additional sites exist.

The CIA declined to comment on the possible second uranium plant. But a former senior U.S. official knowledgeable about North Korea’s clandestine weapons programs said intelligence agencies have long suspected the existence of such a facility, based on numerous independent evidentiary strands.

“The site shown [in 2010] is probably not the only enrichment site,” said the official, who had access to intelligence on North Korea during the Obama administration. The official, who spoke on the condition of anonymity to describe sensitive U.S. assessments, said the intelligence community “does not yet have a complete picture of North Korea’s nuclear program.”

The sudden collapse of the Trump administration’s diplomatic initiative is viewed by analysts as a missed opportunity to fill in some of the most glaring gaps. Although many longtime observers doubted that North Korea would immediately surrender its nuclear stockpile, some saw an opportunity to coax Pyongyang into at least partially pulling back the curtain, including possibly assenting to inspections by international nuclear experts for the first time since 2008.

Currently, U.S. analysts and weapons experts can offer only educated guesses about the number of nuclear bombs North Korea possesses — unclassified estimates put the size of the arsenal at between 20 and 100 warheads by the end of the decade. Also unclear is precisely how much enriched uranium and plutonium North Korea has acquired, and whether its scientists have perfected a reentry vehicle that could reliably deliver a warhead to the U.S. mainland.

Questions also linger over the size and nature of North Korea’s chemical weapons arsenal and whether Pyongyang has manufactured actual weapons from its known stocks of bacterial strains that cause anthrax and the plague.

In addition, U.S. officials would like to press North Korea about its participation in international weapons-trafficking networks, including its procurement of Soviet-era missile designs and its known attempts to sell nuclear and chemical weapons technology to Syrian President Bashar al-Assad. In its initial statements expressing willingness to engage in talks with President Trump, North Korea offered to halt the country’s lucrative trade in weapons technology to foreign governments. With the cancellation of the summit, it is unclear whether that offer still holds.

“One issue that no one is talking about is proliferation,” said Jeffrey Lewis, director of the East Asia Nonproliferation Program at the Middlebury Institute of International Studies in Monterey, Calif. “North Korea sold a nuclear reactor to Syria for the express purpose of making nuclear weapons. That could happen again.”

A partially built, North Korean-engineered nuclear reactor near the Syrian city of Deir al-Zour was destroyed in an Israeli military strike in 2007.

The possibility of a second uranium-enrichment facility has gained credence among Western intelligence agencies in recent months as new information has appeared to bolster the initial reports from the North Korean defector, according to three officials familiar with the evidence.

The computer slide presentation prepared by the Institute for Science and International Security said the facility was believed to be called Kangsong, although different names may have been used in the past. The author of the presentation is David Albright, the institute’s president and a former United Nations weapons inspector who has worked closely with several Western governments in developing assessments of North Korea’s capabilities. A copy of the presentation was provided to The Washington Post.

Albright said a possible location for the alleged plant has been pinpointed by intelligence agencies but is not yet publicly known. The facility was said to be a large building with a capacity for 6,000 centrifuges, the fast-spinning machines used to make enriched uranium for both nuclear power plants and nuclear bombs. Albright said intelligence agencies had already postulated the existence of the plant, in part because of North Korea’s known efforts to purchase key components.

While defector accounts are not always reliable, “this site appears to have received greater governmental scrutiny and credibility” than other reported clandestine facilities, Albright wrote in the presentation.

Officials from two European governments — both of whom spoke on the condition of anonymity to describe confidential assessments shared among intelligence agencies — confirmed in interviews that they had been briefed on the reported enrichment facility and said they viewed the information as broadly credible.

According to Albright, a third European country that studied the evidence has dissented from the consensus view, saying it is not yet convinced that the building in question was designed for uranium enrichment.

“We continue to treat it as an unconfirmed centrifuge plant in our own estimates,” Albright wrote.

“However, unless North Korea reveals its entire centrifuge complex and allows verification,” he added, “any denuclearization agreement will likely be unachievable.”

Mekhennet reported from Frankfurt, Germany.