Way past midnight at an upscale Swiss hotel, negotiations hit the nitty-gritty on a breakthrough deal about Iran’s nuclear program. There was a flurry of calls with the White House, pizza and talk about a tiny, but critical, asterisk in what became the final agreement.

Throughout it all, a band was crooning Irish folk tunes that seemed to grow louder as the tedious negotiations continued into the wee hours of Sunday morning.

On one side of the hotel’s first floor, negotiators talked about centrifuges and uranium, hoping to ink the first step of a comprehensive agreement that could affect the world balance of nuclear weapons technology. Security was tight and an armored personnel carrier was parked outside.

On the other side of the floor, men in tuxedos and women in strapless gowns were partying at a noisy charity event that could be watched from the lobby below.

The final marathon day of negotiations began around 9 a.m. Saturday. Diplomats from Iran, the U.S., Britain, Russia, France, China and Germany as well as the European Union filed into the lobby, their security teams in tow. Their first challenge: negotiating a phalanx of reporters and photographers camped out.

After the arrivals, hours passed with no hint of what was happening behind closed doors.

About 10 hours into the negotiations, Secretary of State John Kerry secretly slipped out of the hotel for about 30 minutes to buy truffles at Auer Chocolatier, a five-generation family business not far from Lake Geneva. The sweets were for his wife and family for Thanksgiving dinner.

The negotiations also paused for dinner. The U.S. delegation dined on different kinds of pasta ordered into Kerry’s suite.

Throughout the day, Kerry felt that there was a chance for an agreement, a senior State Department official told reporters traveling on his plane Monday as it returned from Europe. But there was a point when he became dubious because Iranian Foreign Minister Javad Zarif “looked anxious and appeared to be under pressure from Tehran,” according to the official, speaking on condition of anonymity because the individual was not authorized to discuss the talks by name.

The U.S. delegation was in touch with the White House throughout the day. Kerry called President Barack Obama mid-afternoon local time to update him on the discussions and once again before midnight. After that call, the U.S. team ordered pizza and Kerry broke out some of his chocolates to share with the group.

Kerry was not completely convinced that a deal was going to be reached until well after midnight. At their final three-way meeting, Kerry, Zarif and the European Union’s top diplomat, Catherine Ashton, managed to work through some — though not all — of the remaining issues, the official said.

“The last meeting … was pretty much make or break,” the official said. The agreement, however, was not actually struck at that meeting, according to the official, who declined to provide details on exactly when the deal was sealed.

An official with the group of the six world powers said that toward the end, Russia and China were willing to sign an agreement that had less stringent language. The official, who was not authorized to publicly disclose by name details about the negotiations, said that left the U.S. pressing past midnight — and sometimes alone — for tougher restrictions on uranium enrichment and a heavy water reactor that Iran is building in Arak, southwest of Tehran.

Heavy water reactors produce plutonium, which also can be used to make nuclear weapons — something Iran has long said it has no plans to do.

The negotiations were so detailed that there was even discussion about an asterisk, found on the fourth and final page of the document’s preamble. It states that going forward, the principle of “nothing is agreed until everything is agreed” applies. It means that in the future, as Iran’s nuclear work continues, everything must be agreed upon by all parties.

As Saturday became Sunday, the lobby bar closed, but the band played on. The musicians were belting out the Irish ballad “Danny Boy,” and partiers were doing their best to line-dance in their finest.

The scene below was starkly different. Those waiting for the talks to end were dozing on couches, making the lobby of the swank hotel look more like a bus depot. Phone and laptop chargers were piggy-backed into every available electrical outlet, making sure they were juiced-up and ready to relay news of a deal to the world.

Shortly before 2 a.m., there was a flurry of activity on one side of the lobby. The Iranian news agency ISNA had just reported that the negotiators had resolved their differences. There was a mad dash to find anyone who spoke Farsi. Other Iranian news agencies sent out similar reports. Depending on the translator, a deal had been reached, or a deal was close at hand.

The U.S. delegation played down the reports, saying foreign ministers still had to meet, and advised reporters to stay tuned and drink coffee to stay awake.

It would be another hour — the 18th and last hour of negotiations — before it was official. Michael Mann, a spokesman for Ashton, the EU’s top negotiator, tweeted around 3 a.m. that the parties had “reached agreement.”

(AP)