On the day marking four decades since Islamist revolutionaries orchestrated the takeover of the U.S. Embassy in Iran -- and the prolonged hostage crisis that followed -- the Trump administration announced Monday a major increase in reward money for information on the whereabouts of a retired FBI agent missing in the country for over a decade.

Secretary of State Mike Pompeo announced the State Department's Rewards for Justice Program is now offering an additional $20 million for information leading to the location, recovery and return of Robert Levinson. Combined with a $5 million reward already in place from the FBI, this makes a total of $25 million available.

"The Trump administration has made clear that the regime in Iran must release all missing and wrongfully detained Americans, including Robert Levinson, Xiyue Wang, Siamak Namazi, and others," Pompeo said in a statement. "We will not rest until they are reunited with their families."

Levinson, if alive, is the longest-held hostage in U.S. history.

FAMILIES OF IRAN HOSTAGES, ROBERT LEVINSON CALL ON WORLD GOVERNMENTS TO STEP UP AND ‘SEND OUR LOVED ONES HOME’

Levinson retired from the FBI in 1998. He was 58 years old and working as a private investigator when he traveled to Kish Island, Iran, on March 8, 2007 as part of an unauthorized CIA mission. He was allegedly investigating cigarette smuggling -- and possibly working on a book -- but after checking into the Maryam Hotel, he met with an American fugitive targeted by the CIA for recruitment, a source close to the Levinson case told Fox News in 2016.

Dawud Salahuddin, also known as David Belfield and Hassan Abdulrahman, is still wanted for the 1980 murder of an Iranian diplomat in Maryland. But Levinson was hoping a successful mission that delivered Salahuddin as a CIA asset would lead to Levinson gaining full-time employment with the agency, the source told Fox News at the time.

But then, Levinson disappeared.

The last affirmed proof of life, photos that showed Levinson dressed in an orange jumpsuit, emerged in late 2011. Despite those images, Iranian officials have insisted they have no knowledge of him or his whereabouts.

Levinson's family, however, holds Tehran responsible for his disappearance. Family members said that Iran has also claimed they are not holding Levinson as a political prisoner, even though the country's judiciary division has told the United Nations and the family's Iranian lawyer there is an open case in its files about Levinson.

"This is the 40th anniversary of the day in 1979 when 52 Americans were taken hostage and held for 444 days. Bob Levinson has been held more than 10 times longer - for 4624 days," the family said in a statement to Fox News. "Bob Levinson must come home, and Iran's hostage-taking as government policy must end."

The increase in the reward for information on Monday was dramatic compared to prior amounts. A $1 million reward for information in 2012 was increased to $5 million in March 2015 by the FBI. The Levinson family praised the increased reward on Monday, saying it showed the Trump administration's "commitment" to the mission to bring him home.

"This sends a clear message from our government of how important it is that Bob Levinson be returned to his family and friends who love him," the Levinson family said in a statement to Fox News. "All the Iranian authorities need to do is send him home."

SON OF ROBERT LEVINSON, EX-FBI AGENT MISSING IN IRAN: 'TRUMP CAN GET HIM HOME'

The Levinson family has asked anyone who may know where he is or have information that will bring him back to the U.S. to contact the nearest U.S. embassy or consulate, or email info@rewardsforjustice.net.

"All we care about is having him home, so he can live the rest of his life in peace," the family said.

In addition to increasing the reward money, the Trump administration on Monday slapped new sanctions on nine officials who are among the core supporters of Iran's Supreme Leader, Ali Khamenei.

Among those sanctioned include Khamenei’s appointees in the Office of the Supreme Leader, the Expediency Council, the Armed Forces General Staff and the Judiciary, according to the State Department.

"The designation seeks to block funds from flowing to a shadow network of Khamenei’s military and foreign affairs advisors who have for decades oppressed the Iranian people, supported terrorism, and advanced destabilizing policies around the world," Pompeo said in a statement.

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The actions also come as Iran disclosed it was doubling the number of advanced centrifuges it operates, further trimming the time experts estimate Tehran needs to have enough material to build a nuclear weapon.

Fox News' Cristina Corbin, Hollie McKay and Greg Norman and The Associated Press contributed to this report.