Han Omar Sachedina has been accused by the U.S. State Department of being a major agent for Iran’s nuclear program. Mr. Saeed was denied entry into the United States, his family says, and is currently detained. He is charged with being part of the Iranian-backed military group called the Fatemiyoun, which is headed by an accused terrorist, Major General Qassem Soleimani.

In a statement, the U.S. State Department said Iranian officials “infiltrated our Embassy on July 12 to pose as U.S. Embassy officials to gain unauthorized access to American citizens.”

“On July 12, 2009, this is what happened. American citizens were forced against their will to divulge sensitive military secrets held by the United States,” it added.

After being denied entry, Mr. Saeed was released a few days later. Iran’s Islamic Revolutionary Guards Corps (IRGC-QF), the military’s elite paramilitary force, posted a photo of the young diplomat with the caption “Reza Qudiho, ambassador, released in front of [Iranian] military bases.”

Mr. Hamidi said his uncle was held for three days under threat of “being forced into the brigadoon” in Iran’s Evin Prison, which has a history of holding U.S. citizens. The government of Iran, he said, has threatened to torture him.

“I wanted to get back to Pakistan, but when I saw in front of me a soldier holding me, I didn’t know where to go,” Mr. Hamidi said.

Mr. Hamidi said the government tried to get his uncle to sign a pledge pledging his allegiance to Iran’s Supreme Leader Ayatollah Ali Khamenei while he was still being held captive and would kill his mother to silence him if his uncle tried to leave.

“They demanded he sign the pledge and if he tried to leave he would be killed as well,” Mr. Hamidi said. “The next night they threatened to kill my mother and I was afraid.”

After being freed, Mr. Hamidi called his mother from an Iranian prisoner of war camp and she told him his uncle was dead, his brother said.

He was released two days later after being brought to the U.S. Embassy in Islamabad, Pakistan.

The State Department statement said Mr. Soleimani had ordered his Iranian agents