Built nearly 18 years ago to detain suspected terrorists on a U.S. naval base in Cuba, Guantanamo Bay has grown into what seems to be the most expensive prison on earth.

The U.S. government has spent an estimated $161.5 million housing the suspected mastermind of the 9/11 terror attacks – Khalid Sheikh Mohammed. And up until Saturday, Mohammed was to receive a coronavirus vaccine so that he could be tried and put to death, if convicted.

Captured in 2003, Sheikh Mohammed confessed to being a mastermind behind some of the most prolific terror attacks in the past few decades, most notably the 9/11 attacks.

Mohammed's death penalty trial was originally set for Jan. 11, 2021, but it was delayed due to the pandemic.

A lack of vaccinations had reportedly made it difficult for federal prosecutors to move forward with war crimes hearings at the base, which is why Terry Adirim, the principal deputy assistant secretary of defense for health affairs under President Biden, signed an order Jan. 27 to vaccinate the suspected terrorists, a Pentagon spokesperson told the New York Post. Two defense officials confirmed the plan to Fox News.

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The Defense Department reversed course Saturday. "No Guantanamo detainees have been vaccinated," Defense Department press secretary John Kirby tweeted. "We're pausing the plan to move forward, as we review force protection protocols. We remain committed to our obligations to keep our troops safe."

This February 2017 photo provided by his lawyers shows Khalid Shaikh Mohammad in Guantanamo Bay prison in Cuba. (Courtesy Derek Poteet via AP)

It's not exactly clear how much the federal government spends housing its Gitmo prisoners, but it's somewhere between $9.5 and $13 million per prisoner, per year. The prison currently has 40 inmates. That's compared to $78,000 spent per inmate at a "supermax" prison in Florence, Colo., home to some of the highest-risk prisoners in the U.S.

Using the $9.5 million figure NPR reached in a 2020 report, U.S. spending in the 17 years Mohammed has spent at the prison has topped $161 million.

Guantanamo has reportedly cost U.S. taxpayers over $6 billion since its inception. Included in that figure are charter planes to and from the island with few passengers, hundreds of thousands' worth of government devices that are destroyed each year to spills of classified information, Pentagon-funded defense attorneys priced at half a million dollars per year and total legal costs amounting to $60 million, even though Guantanamo has only ever had one finalized conviction.

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But the New York Times tally edges the number up to $13 million per prisoner, per year. "I think it's crazy," former President Trump said of the cost.

About 770 foreign men and boys have been held captive as wartime prisoners at Guantanamo, with prison population peaking at 677 in 2003. The last prisoner to arrive at the island came in 2008.

The Bush administration, which opened the prison in the aftermath of 9/11, released about 540 detainees, mostly repatriating them back to Pakistan, Afghanistan and Saudi Arabia. The Obama administration released another 200.

Trump pointed out that his predecessor had tried unsuccessfully to shutter the facility. "Look, President Obama said that Guantánamo Bay would be closed, and he never got it done." Obama had been blocked by Congress from transferring the 40 inmates to prisons on U.S. soil, as members didn’t take kindly to the idea of terrorists on U.S. soil.

President Biden, who throughout his campaign supported the shutdown of the center, was asked last year as a Democratic candidate why the Obama administration hadn’t been able to do so.

"You have to have congressional authority to do it. They've kept it open," Biden said at a December 2019 debate. "We, in fact, think... it is an advertisement for creating terror."

In 2019, a top attorney there filed a whistleblower complaint against the prison alleging "gross financial waste" and "gross mismanagement."

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The cost has risen dramatically over the years – a 2013 Defense Department report calculated the per-prisoner detention cost at only $2.7 million.

Capt. Brian L. Mizer, a Navy lawyer who has represented Guantanamo detainees over the years, called the prison "America's tiniest boutique prison, reserved exclusively for alleged geriatric jihadists."