WASHINGTON – The Trump administration has used diplomatic pressure, legal action, economic sanctions – and even cold, hard cash – to try to get its hands on a hulking Iranian oil tanker that has been spinning its way around Africa and the Middle East for months.

The extraordinary effort to seize the vessel has come to naught – so far. Even a curious State Department offer to make the ship’s captain a multi-millionaire fell flat.

But the cat-and-mouse game between Iran and the Trump administration over the vessel – called the Adrian Darya 1 and laden with 2.1 million barrels of oil – is emblematic of an increasingly confrontational relationship. And like the fate of the supertanker and its crew, the outcome of the U.S.-Iran tensions remains unclear.

The Trump administration’s efforts to capture the Adrian Darya is a small part of its “maximum pressure” campaign – aimed at reducing Iran’s oil exports to zero, strangling its economy, and forcing its leaders into negotiations with President Donald Trump. Trump withdrew the U.S. from the 2015 nuclear deal between Iran and other world powers, saying it did not do enough to curb the Islamic Republic's ballistic missile program and support for terrorism.

Experts say Iran’s ability to keep the Darya out of the U.S. government’s long reach illustrates the shortfalls of the U.S. strategy. And it comes as Iran leaders once again rejected negotiations with Washington, saying Trump must lift U.S. sanctions first.

On Saturday, Iran further reduced its compliance with the nuclear deal, saying it has begun injecting uranium gas into advanced centrifuges and that the country will no longer abide by the deal's limits on its nuclear research and development.

“The Iranians are not capitulating,” said Barbara Slavin, director of the Future of Iran Initiative at the Atlantic Council, a foreign policy think tank in Washington. “They’re not saying ... ‘Please, Mr. Trump, can we have a meeting with you?’”

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Instead, Iran has launched its own aggressive strategy, downing an American drone, allegedly sabotaging other ships passing through the Strait of Hormuz, and using circuitous shipping routes and cloaked transponders to move its own oil.

The Adrian Darya – previously named Grace 1– began its current journey in mid-April, starting in Iran's main export terminal where it apparently was loaded up with light crude oil, said Samir Madani, co-founder of TankerTrackers.com, a company that uses satellite imagery and other tools to track crude oil shipments. The ship's transponder was "cloaked" at the time, he said, and his firm couldn't get any images of it because of bad weather.

"She resurfaced then, heading back out of the Iran area but waited around in the Persian Gulf until around May," he said. "Then she left, sailing all the way around Africa" and apparently heading to the Mediterranean.

The two nations’ competing playbooks collided in July near Gibraltar, when the British Royal Navy seized the Adrian Darya, previously called the Grace 1. British and American officials suspected the ship was headed to Syria, in violation of European sanctions on oil sales to the brutal Assad regime in that war-torn country.

The Trump administration tried to seize the vessel from Gibraltar, saying the ship and its oil were subject to U.S. forfeiture based on alleged violations of bank fraud and money laundering statutes, and other crimes.

But officials in Gibraltar defied the U.S. legal move and released the oil tanker on Aug. 16. The ship’s captain, a 43-year-old Indian man named Akhilesh Kumar, steered the supertanker slowly away from Gibraltar and into international waters, according to Madani.

About a week later, as the Adrian Darya meandered toward the east Mediterranean, Kumar received a remarkable email.

“This is Brian Hook … I work for secretary of state Mike Pompeo and serve as the US Representative for Iran,” the Aug. 26 missive read. “I am writing with good news.” Hook confirmed to USA TODAY that he sent the email, which was first reported by the Financial Times.

Hook proceeded to offer the ship’s captain several million dollars, if he agreed to steer the vessel to a port where the U.S. could seize it.

“With this money you can have any life you wish and be well-off in old age,” Hook wrote in a second email. “If you choose not to take this easy path, life will be much harder for you.”

The captain apparently did not respond to Hook’s email. And on Aug. 30, the U.S. Treasury Department sanctioned the ship and Kumar. USA TODAY was unable to contact Kumar for comment or to confirm that he read the emails.

Iran's semi-official news agency labeled the move an attempted "bribe" and the country's foreign minister, Javad Zarif, derided it as desperate.

"Having failed at piracy, the US resorts to outright blackmail—deliver us Iran’s oil and receive several million dollars or be sanctioned yourself," Zarif tweeted on Wednesday.

Hook’s offer of U.S. taxpayer funds is allowed under a State Department program called Rewards for Justice, which provides money to individuals who help the U.S. prevent terrorist attacks or catch perpetrators. Hook publicly announced this week that the State Department would award up to $15 million to anyone who helped the U.S. disrupt the financial operations of Iran’s Revolutionary Guards Corps, an elite military unit that the Trump administration has designated as a terrorist group.

“It’s the first time that the United States has offered a reward for information that disrupts a government entity’s financial operations,” Hook told reporters. “We have taken this step because the IRGC operates more like a terrorist organization than it does a government.”

Slavin said Hook’s email to Kumar was “amateurish” and unprecedented.

“I have never seen anything like that in my life,” she said.

“It really did read like a Nigerian come-on: Send me your bank account information and you will become a millionaire tomorrow,” Slavin added, referring to the notorious foreign email scams. “Whose idea it was, I can’t even imagine.”

As of Sept. 4, the Adrian Darya was approximately 60 miles off the Syrian coast, according to TankerTrackers.com. Satellite imagery tweeted out Friday by Trump's National Security Advisor John Bolton appeared to show the ship just a few miles off the coast of Syria. It was not clear if its oil cargo had been unloaded.

Madani, the company’s co-founder, said the crew may be planning to offload some or all of its oil, either via a Syrian port or a ship-to-ship transfer, and then head through the Suez Canal and back to Iran. He said he’s not sure why the Trump administration seems so focused on this particular ship, but its pressure has not stopped Iran from sending oil to Syria via other ships and other routes.

The U.S. is unlikely to be in a position to impound the vessel any time soon, said Andrew Serdy, an expert on maritime law at the University of Southampton in southern England.

"The boat can't be seized in a foreign nation's territorial sea,” he said. The only place that the U.S. could realistically seize the Iran-flagged Adrian Darya1 would be in its own territorial waters several thousand miles away: the U.S. East Coast.

In the meantime, Iran on Saturday said that it seized a separate tugboat near the Strait of Hormuz, a key waterway for oil transportation. It said 12 Filipinos were aboard the boat. It was not immediately clear what national flag the boat was sailing under.