This image provided by the FBI shows a photo of former FBI agent Robert Levinson, who went missing on Kish Island, Iran, on March 9, 2007, shackled and holding a sign. (FBI via AFP/Getty Images)

As the families of Americans celebrate the release of their loved ones held in Iran, the authorities in Tehran said they would not be freeing a businessman arrested in October and were silent on the fate of a former FBI agent who disappeared in the country.

It was unclear why Siamak Namazi, 44, an Iranian American based in Dubai, was arrested in October while visiting a friend in Tehran where he had done consultant work over the previous decade. Namazi is the son of a prominent family in Tehran who couldn’t be reached. Namazi immigrated to the United States in 1983, and he later returned to Iran after graduating from college to serve in the Iranian military.

“I don’t know what’s going on,” said Ahmad Kiarostami, a friend. “I’m still hopeful he’s going to be released in the next few days. That’s what I hope.”

Kiarostami said he had traded Facebook messages with Namazi’s family in Iran, but they didn’t know anything.

He said it was a “big surprise” when Namazi wasn’t freed with the others.

Four Americans and seven Iranians were set to be exchanged in a deal linked to the imminent implementation of a landmark nuclear deal between Tehran and six world powers. Here's what we know about who they are. (Jason Aldag/The Washington Post)

U.S. officials said Saturday that they would continue to talk with Iran to secure the release of Namazi as well as to obtain information about the whereabouts of Robert Levinson, 67, who went missing on an Iranian island in March 2007.

News that Levinson had not been freed left his family distraught.

“Of course we are happy for those families, but angry and devastated,” Suzanne Halpin, the sister of Levinson’s wife, said in an email.

The Levinsons have hoped for years that their father would eventually be released after a deal was reached to limit the Iranian nuclear program.

The family thought the United States had “squandered its best opportunity for leverage in ensuring my father’s safe return home,” Levinson’s son, Daniel, wrote late last year in The Washington Post, after Iran and six world powers struck a nuclear deal.

It was an ominous sign that he wasn’t released with other the Americans, officials said.

Washington Post journalist Jason Rezaian has been freed from prison, according to U.S. and Iranian officials. He was arrested in Iran in 2014 and convicted of espionage last year. Here's what you need to know about the case against him. (Monica Akhtar/The Washington Post)

“They were given every opportunity to save face,” a U.S. intelligence official said Saturday. “We are still not giving up.”

The Iranians have never acknowledged holding Levinson, and some former and current U.S. intelligence officials fear that he might be dead.

“Discussions with the Iranians have focused on trying to discern his whereabouts and bring him home,” said a senior administration official, speaking on anonymity guidelines set by the administration. “We weren’t able to do that in that time frame and we have agreed with the Iranians that we will continue to use the channels that [now] exist for that purpose.”

A second administration official said that agreement over the other prisoners provides “an opportunity to focus specifically on identifying [Levinson’s] whereabouts. . . . We recognize that his family has endured the hardship of his disappearance for over eight years now, and we will not cease our efforts” until he is returned home.

Levinson joined the FBI’s New York field office in 1978 after spending six years with the Drug Enforcement Administration. He was an expert on the New York mob’s five families. Eventually, he moved to the Miami office, where he tracked Russian organized-crime figures and developed a reputation for developing sources.

After retiring from the FBI in 1998, Levinson worked as a private investigator and as a CIA contractor. Levinson was supposed to produce academic-like papers for the agency but was operating more like a spy, gathering intelligence for the CIA and writing numerous well-received reports, officials said.

Levinson traveled the globe. He went to Turkey and Canada, among other countries, to interview potential sources, sometimes using a fake name. But CIA station chiefs in those countries were never notified of Levinson’s activities overseas, even though the agency reimbursed him for his travel, a violation of the rules.

On March 8, 2007, Levinson flew from Dubai to the Iranian island of Kish and checked into a hotel. He entered Iran to gather information about government corruption.

He met with Dawud Salahuddin, a fugitive wanted for the murder of an Iranian dissident and diplomat who was shot at his house in Bethesda, Md. Levinson thought Salahuddin could supply details about the Iranian regime, perhaps information that could interest the CIA, according to officials who have reconstructed some of his movements.

Levinson spent hours talking to Salahuddin. The next morning, Levinson checked out of his hotel and vanished, officials said. The United States suspected the Iranian security services were behind his abduction, according to a diplomatic cable disclosed by WikiLeaks.

The FBI has offered $5 million for information leading to his safe return.

The CIA ultimately concluded that it was responsible for Levinson while he was in Iran and paid $2.5 million to his wife, Christine, former U.S. intelligence officials have said. The CIA leadership disciplined 10 employees, including three veteran analysts who were forced out of their jobs, the officials said, because Levinson was run by people who had no authority to manage operations overseas.

The last proof of life came about three years ago when the Levinson family received a 54-second video of him and later pictures of him shackled and dressed in an orange jumpsuit similar to those worn by detainees at the U.S. military prison at Guantanamo Bay, Cuba.

“I have been held here for 3 1/2 years,” he said in the video. “I am not in good health.”