Last week, a leaked email showed that the US government offered millions of dollars to the captain of an Iranian oil tanker in exchange for letting the ship be captured.

It opened a window onto a species of US covert operation which has run for years but rarely been known to the public.

In defense and security circles, many were incredulous that the offer would be committed to email.

"How f---ing stupid are they?" asked one source, suggesting that the leak will do damage beyond one embarrassing news cycle.

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Last week the ongoing saga of US attempts to hinder the passage of an Iranian-flagged oil tanker traversing the Mediterranean took a bizarre turn, when it became public knowledge that the ship's captain was offered a large payment by the US government to defect.

Although unusual in many respects, the offer brought to light a practise that many consider an open secret: that the US will pay key individuals millions of dollars to betray its enemies.

The offer came to the captain of Adrian Darya-1, a tanker which was detained by UK forces in the Mediterranean, but later let go.

After its release, an email from State Department official Brian Hook, leaked to the Financial Times, exposed an offer from the US government. The captain was told he would secure millions of dollars, and a new life in the US, if he would sail the ship to a port where it could be recaptured.

Brian Hook, US Special Representative for Iran, in London on June 28, 2019. Simon Dawson/Reuters

'How f---ing stupid are they?'

The initial response to the leak in defense and security circles was disbelief, and even laughter — not at the offer itself, but the fact that a US official had ever expressed it in writing.

One source, the owner of a London-based business intelligence firm who has worked with the US on similar operations in the past, was among those who were incredulous.

"I mean to send an email, ffs," he said, using the abbreviation for a common vulgar expression of annoyance.

The executive asked not to be identified because of ongoing client operations and fears of being identified.

"How f---ing stupid are they? They should have sent a covert team on to disable the tanker or found enough dirt on the captain to f--- him over," said the source.

He argued that making the policy overt in an email risked compromising the entire program. Which it did.

Iranian oil tanker Adrian Darya 1, previously named Grace 1, anchored in the Strait of Gibraltar on August 18, 2019. Jon Nazca / File Photo / REUTERS

The ship itself was blacklisted under a package of sanctions the US has been using to pressure Iran into renegotiating the so-called nuclear deal struck by the Obama administration.

The designation came after the tanker was seized by UK forces off the coast of Gibraltar in July. It was released after six weeks following a court ruling, and after Iran promised not to send the ship to Syria.

The US response focused on ensuring the tanker didn't make it to its apparent initial destination off the coast of Syria, an Iranian ally, to which it was expected to deliver 2.3 million barrels of oil.

The ship is currently moored off the Syrian port of Tartus. Iran claims the cargo had been sold and unloaded to Syria — but monitoring groups have produced satellite images that suggest the oil is still on board.

Read more: The State Department tried to bribe the captain of an Iranian tanker with several million dollars, but he headed toward Syria instead

What appears to be the Iranian oil tanker Adrian Darya-1 (circled) off the coast of Tartus, Syria in this September 6, 2019 satellite image. Maxar Technologies/Handout via Reuters; Business Insider

Informing for the US government is lucrative but risky work

In most cases, US intelligence and law enforcement have used paid confidential informants (often referred to by the abbreviation CI) to help identify money laundering operations employed by drug cartels or designated terror groups like Iran's Revolutionary Guard or Hezbollah.

The business intelligence firm owner described a murky world where helping the US government as an informant can be lucrative, but also filled with physical and financial risk.

"Well... they are willing to pay, getting it is another matter," he said.

This, the source said, can depend on the competing interests of different US agencies.

For example, the Drug Enforcement Administration (DEA) might have run a successful operation with an informant.

But, once the operation becomes public, the source said there can be internal interference from other agencies competing to make busts, or who worry about their own assets.

"There are a number of examples where it appears an intelligence agency has interfered or deliberately caused problems for law enforcement in this area," he said.

"Getting it requires being lawyered up well in the USA… and not being shafted."

A composite image of Iranian President Hassan Rouhani and Trump. Michael Gruber/Getty Images; Olivier Douliery-Pool/Getty Images; Samantha Lee/Business Insider

Despite the risks, multiple confidential informants have made millions, according to the source, and confirmed by multiple other former DEA officials.

But the most successful areas for informants seeking payouts has been the battle throughout Central and South America against large drug cartels and rebel groups in league with drug traffickers.

"It works very well with their deep undercover guys and some have made millions," he said.

"Especially the fake FARC ones," he said referring to the Colombian rebel group closely tied to drug trafficking.

In at least three major public cases, the DEA has paid out huge sums to informants after sting operations:

One against the Russian arms trafficker Viktor Bout in 2008.

Another against a Syrian arms trafficker based in Spain named Monzer al Kassar, who was detained in 2007.

Another linked to the 2017 arrest of Hezbollah financier Qassam Tajjadine in Morocco.

All three were eventually extradited to the US and convicted.