DUBAI, United Arab Emirates (AP) — A Princeton scholar held for three years in Iran on widely criticised espionage charges was freed Saturday as part of a prisoner exchange that saw America release a detained Iranian scientist, a rare diplomatic breakthrough between Tehran and Washington after months of tensions.

The trade on the tarmac of a Swiss airport saw Iranian officials hand over Chinese American graduate student Xiyue Wang for scientist Massoud Soleimani, who had faced a federal trial in Georgia over charges he violated sanctions by trying to have biological material brought to Iran.

The swap, however, had clear limits. Crushing US sanctions on Iran blocking it from selling crude oil abroad remain in place — part of President Donald Trump’s maximum pressure campaign imposed following his unilateral withdrawal from Tehran’s nuclear deal with world powers last year. Those sanctions in part fuelled the anger seen in nationwide protests last month that Iranian security forces violently put down — unrest that reportedly killed over 200 people.

Meanwhile, Western detainees from the US and elsewhere remain held by Tehran, likely to be used as bargaining chips for future negotiations. At least two American families of detainees, while praising Wang’s release, questioned why their loved ones didn’t come home as well.

Wang’s release had been rumoured over recent days. One lawyer involved in his case tweeted a Bible verse about an angel freeing the apostle Peter, just hours before Iranian Foreign Minister Mohammad Javad Zarif broke the news in his own tweet. He posted pictures of himself with Soleimani at the Zurich airport before quickly whisking him back to Tehran by jet.

Trump shortly after acknowledged Wang was free in a statement from the White House, thanking Switzerland for its help. The Swiss Embassy in Tehran looks out for America’s interests in the country as the US Embassy there has been closed since the 1979 student takeover and 444-day hostage crisis.

“We’re very happy to have our hostage back. The whole Princeton University community is very thrilled, and it was a one-on-one hostage swap,” said Trump, speaking to reporters outside the White House before he left on a trip to Florida. “Actually, I think it was a great thing for Iran; I think it was great to show that we can do something. It might have been a precursor as to what can be done, but we have our hostage back.”

Brian Hook, the US special representative for Iran, accompanied Soleimani to Switzerland to make the exchange. He later posed for a photograph with Wang, who carried a folded American flag in his arms while wearing gray workout clothes.

Hook and Wang travelled to Landstuhl hospital near Ramstein Air Base in Germany, where Wang will likely be examined by doctors for several days.

Wang’s wife, Hua Qu, released a statement saying “Our family is complete once again.

“Our son, Shaofan and I have waited three long years for this day and it’s hard to express in words how excited we are to be reunited with Xiyue,” she said. “We are thankful to everyone who helped make this happen.”

Soleimani arrived at Tehran’s Mehrabad International Airport with Zarif, where his wife and family wrapped garlands of yellow and purple carnations around his neck. He briefly spoke to journalists from State-run media, his voice shaking and a tear running down his face under a portrait of the late Supreme Leader Ayatollah Ruhollah Khomeini.