It's been nearly 11 years since retired FBI agent Robert Levinson disappeared while investigating a cigarette smuggling ring on Kish Island off the coast of Iran in the Persian Gulf.

When the Obama administration negotiated the release of five American hostages in January 2016 coinciding with the implementation of the landmark nuclear deal, Levinson was not among them.

In 2011, his captors released a proof of life video.

"Please help me get home. 33 years of service to the United States deserves something. Please help me," pleaded Levinson.

One of Levinson’s sons wonders why his father was not among the Americans released two years ago.

“My father was left behind and in an agreement where there was a prisoner swap in the culmination and finalization of the Iran nuclear deal,” said David Levinson in an interview with Fox News.

His family had high hopes when President Trump announced that unless all the Americans were released from Iran the Islamic republic would face new sanctions.

“We are hoping that if President Trump, if he's listening, can apply the appropriate pressure because we know that if he makes this a priority, his skills in negotiation, his willingness to push for these issues can bring him home,” Levinson added.

In July days after the two-year anniversary of the nuclear deal with Iran, the White House issued a statement, “American Citizens Unjustly Detained in Iran.”

Levinson was named along with three other Americans, Xiyue Wang and Siamak and Baquer Namazi.

"President Trump is prepared to impose new and serious consequences on Iran unless all unjustly imprisoned American citizens are released and returned," according to the statement.

After testifying to Congress last year alongside the families of other American hostages, Levinson's wife, Christine, appealed directly to the Iranian government.

"Bob I will continue to do everything I can to bring you home alive so our family can be whole again. We love you and miss you every day," she said while seated next to her son, David Levinson, in a video for the Iranians. The family maintains a Facebook page urging anyone with information to come forward.

Still imprisoned

Robert Levinson is just one of nearly 20 known American hostages who remain in captivity or imprisoned by hostile regimes.

Siamak Namazi, an Iranian-American businessman, was nabbed while visiting his family in October 2015, three months after the Iran nuclear deal was signed.

His father, Baquer, a former UNICEF diplomat, was arrested in February 2016 after the Iranian authorities granted him permission to visit his son in the notorious Evin Prison in Tehran.

"My father, Baquer Namazi, was lured back to Iran from a brief trip abroad by the promise of seeing Siamak, but instead he was also detained," said another son, Babak, during congressional testimony last July.

Now both are behind bars. Namazi's father is 81 years old.

Iran also detained Xiyue Wang, an American grad student from Princeton who was conducting research for his Ph.D. dissertation. His wife, Hua Qu, and 4-year-old son live in New Jersey and are trying to remain strong thinking of him suffering in Iran's Evin Prison.

"He has done nothing wrong. He is completely innocent. This is a tragic mistake for him and my family," Qu said in an interview with Fox News from China. “He is a history nerd. He is not a spy."

“He was doing this research only because he grew a long standing respect for Islam and his love of history,” she added.

“I hope the U.S. government can bring Iran to a dialogue to resolve my husband’s case as quickly as possible to bring him home ... before my son’s fifth birthday.”

Qu said she receives daily calls from prison, where he has been held for over 18 months. She said her husband complains of bed bugs and is deprived of sleep because of the “very poor” conditions.

More than a dozen others

There are more than a dozen other Americans held in North Korea, Turkey, Afghanistan, Syria, Mali, and Yemen and Venezuela.

Gholamrez "Reza" Shahini, Karan Vafadari and Nizar Zakka are three other Americans being held in Iran.

North Korea is still holding a 62-year old missionary from Virginia, Kim Dong Chul, and two American professors, Kim Sang-duk and Kim Hak Song, who were teaching inside the rogue communist regime.

In Turkey, American pastor Andrew Brunson of North Carolina is being held by the Turkish government along with a NASA scientist arrested on vacation, Serkan Golge, who was arrested in July 2016.

In Afghanistan, American University professor Kevin King is still being held by the Taliban. American writer Paul Overby was captured three years ago.

Journalist and former U.S. Marine Austin Tice is thought to be held by the Syrian regime. He's been missing nearly five years.

In Mali, aid worker Jeffry Woodke was taken hostage by Al-Qaeda in 2016.

And Danny Burch, an oil worker from East Texas, was abducted at gunpoint in Yemen in September.

Pleading for help

Laurie Holt's son, Josh, a Morman missionary from Utah, is imprisoned by the Venezuelan government on trumped-up charges of weapons smuggling.

“I'm very dizzy and I can't think and my stomach hurts me, superbad, I really don't know what to do,” her son told his mother in a recorded telephone call.

Josh’s mother is pleading for help and made a video appeal to President Trump after he was elected.

"President Trump, my son's only crime was being an American citizen," she said.

Her 25-year old son went to Venezuela to get married. He and his wife have been held by the authorities as a political bargaining chip for more than a year.

His mother spoke recently to Fox News Channel’s Shannon Bream.

"Josh sounds like he's on his deathbed to me. That is not my Josh, it's his voice but he is pleading for help and I don't know how else to get him, I hope that to go to the public and put the pressure on our government to do something, do something more than what you have done so far, because obviously, it's not working," she said.

The case has the attention of Sen. Orrin Hatch, R-Utah.

"So far we've struck out we are doing everything we possibly can to get him out of there. His parents are wonderful, humble people. We're still working on it, but we've had a lack of success,” he told Fox News this week on Capitol Hill.

Data remain classified

The U.S. government will not publically disclose the number of American citizens being held hostage and the data remain classified over security concerns. Some of the cases have not been made public. The State Department says it has successfully aided the release of nearly 200 hostages since 2015.

“The U.S. government currently has less than 20 active cases that fall under the authority of PPD 30 (Presidential Policy Directive --Hostage Recovery Activities put in place in June 2015),” said State Department spokesperson Heather Nauert.

The Americans are being held by terrorist groups, criminal organizations, as well as regime states.

After James Foley was executed by ISIS in a gruesome video released to the public in August 2014, then-President Obama ordered a review of U.S. hostage policy, which led to the directive a year later.

But some officials working the hostage issue for years are frustrated that Secretary of State Rex Tillerson has not named a new hostage envoy.

The position has been vacant for nearly a year.