He handed the prostitute a key and sent her on ahead alone towards the house. She was a canary. If the house was under surveillance, he hoped she would spook the watch team into giving themselves away and he could slip out unnoticed.

But the young woman made it without incident and minutes later Samer El Debek found himself inside the house, looking at dozens of boxes containing what he believed to be ammonium nitrate, a precursor for powerful explosives.

It was May 2009 and Debek was in Bangkok, Thailand on his first official "mission" for the Iran-backed Lebanese militant group Hezbollah, according to a criminal complaint unsealed in early June and allegedly based mostly on Debek's own account to FBI agents. Last month, an attorney for Debek declined to comment for this report.

The complaint says that Debek, who was about 29 years old at the time of the Thailand mission, had been recruited into Hezbollah about two years before and over nearly a decade would receive military-style training in firearms, RPGs, explosives, surveillance and counter-surveillance. An FBI bomb technician who later spoke to Debek said he was impressed with his "high degree of technical sophistication" when it came to bomb-making.

His missions would take him from Thailand to Panama before he eventually attempted to settle with his deadly skills in Michigan. Debek, a naturalized U.S. citizen, was arrested there, in Livonia, Mich. on June 1.

The complaint against Debek, along with another purported Hezbollah operative arrested in New York named Ali Kourani, paint a detailed, darkly colorful picture of the time, effort, tradecraft and training Hezbollah allegedly devotes to what officials call its "sleepers" abroad -- molding what experts say are highly competent and more treacherous operatives than the ISIS-inspired attackers who dominate the headlines more frequently.

"These guys are definitely not your guy that walks into a nightclub in Orlando or these other individual attacks you see," said former FBI special agent Brad Garrett, referring to the June 2016 shooting at the Pulse nightclub in Orlando, Florida by a young man who pledged his allegiance to ISIS. "These guys obviously have received formal training, which potentially makes them a lot more dangerous."

"They don't look like wannabees," Bob Baer, a former CIA officer and expert on Hezbollah, said in June of Debek and Kourani. "How these guys operate... they seemed a lot more legit."

Baer said the arrests were the latest indications that Hezbollah, and by extension its backer Iran, have been preparing the ground for potential attacks wherever they have interests around the globe, including in the U.S.

"The fact [is] that there are [Hezbollah] cells in the U.S. and if we go to war with Iran... will these guys go operational?" Baer said, echoing a 2012 House Homeland Security Committee report that estimated Hezbollah could have "hundreds" of operatives in the U.S., though the actual number is hard to know "because of their operational security expertise." 

In the end, Debek's first mission in Thailand was more like a highly-combustible errand. He was first ordered to remove the ammonium nitrate from the house and then told to put it back, the complaint says. 

But Debek and his handlers purportedly took his tradecraft seriously. He used two real passports to get to Bangkok from Lebanon, switching during a stop in Malaysia. He established "contacts" with women online in Thailand months before in order to build up his cover story as a sex tourist, in case local authorities came asking.

More than anything, the mission was meant to be simple but potentially dangerous, according to Baer, who described it as Hezbollah's "forged in the fire kind of thing."

"Sending these guys out there to see how they operate overseas would make sense from a tactical standpoint," Baer said.

Around the same time Debek was slipping into Thailand, New York City resident Ali Kourani received good news he had been waiting on: His U.S. citizenship application had been approved. With his newfound status as an American, Kourani immediately filed for a U.S. passport, which was also granted.

What American immigration officials apparently didn't know was that Kourani had been involved with Hezbollah since he attended one of the group's "boot camps" as a teenager in Lebanon in 2000. Like Debek's, this account is based on a criminal complaint filed in late May purportedly much based on Kourani's alleged conversations with the FBI. A public defender assigned to Kourani did not respond to requests for an interview last month and a newly hired attorney recently declined to comment on the allegations against his client.

The complaint says Kourani first came to the U.S. in 2003 but never forgot his Hezbollah ties. Two years after his home was destroyed in a bombing during the Lebanon War in 2006, Kourani said he was recruited into Hezbollah's Islamic Jihad Organization. The complaint described the IJO as the part of Hezbollah "responsible for the planning and coordination of intelligence, counterintelligence, and terrorist activities on behalf of [Hezbollah] outside of Lebanon." The organization itself has more than one name, including the "External Security Organization" and simply "910."

Kourani allegedly put it more succinctly to law enforcement officials: The IJO conducts "black ops" on behalf of Hezbollah and "the Iranians." Kourani said he was to be a "sleeper" in the U.S., the complaint says.

From 2005 to 2015, the complaint says, Kourani returned to Lebanon approximately every year and was trained in the finer arts of espionage, interrogation and terrorism. He kept in touch with his Lebanon-based handler called only "Fadi" using coded language over email. Any mention of a "bride" would be an order for Kourani to come back to Lebanon, for instance.

In order to get in touch with Fadi when Kourani was in Lebanon, he would call a certain phone number and say a pre-arranged code to identify himself. Fadi would then call a phone belonging to one of Kourani's family members and set up an in-person meeting. Even then, Fadi often wore a mask, the complaint says. Early in their relationship, Fadi explained the "golden rule" to Kourani: The less you know, the better.

Starting as early as 2011, Kourani was directed to gather information on facilities in the New York area, including government and military buildings and major airports. He was also tasked with identifying New York-based members of the Israeli military, perhaps, he believed, for a retaliatory attack after the assassination in 2008 of IJO leader Imad Mughniyah.

As Kourani allegedly carried out Hezbollah's work, he also blended into the American backdrop. He earned a college degree in biomedical engineering and then a Masters in Business Administration. He kept his head down and understood that as a sleeper, he could be "activated" at any moment.

Meanwhile, in 2011 and 2012, Debek was twice ordered to go to Panama to conduct surveillance there on the Panama Canal and the Israeli Embassy. He was also to locate the U.S. Embassy. During his second trip, Hezbollah wanted to know if it would be possible to get to a ship in the canal, the complaint says. As his cover, Debek was to pose as a businessman exploring potential opportunities in Panama. To get close to the canal, the FBI alleged emails also indicate Debek considered joining a local tour group or even pose as a birdwatcher at a nearby park.

Debek succeeded in getting photographs of the canal, which the FBI says he turned over to Hezbollah, but did not take photographs of the Israeli or U.S. embassies.

Then, in 2015, Debek and Kourani's undercover work for Hezbollah began to unravel.

In mid-September of that year, Kourani's clever tradecraft failed him. Coming through New York's JFK airport, Kourani attempted to hide his cell phone's SIM card under a travel sticker in his passport. The trick impressed Baer, the former CIA officer, since it would mean customs officials likely would be holding the passport themselves during any search of Kourani's person or belongings and might forget to look hard at was already in their hands. But an alert American official spotted the chip anyway. Surprisingly, the chip was not searched and Kourani was allowed to go, the complaint says.

It was around this time that Kourani claims he was "deactivated" by Hezbollah.

In December 2015, Debek claimed he was detained in Lebanon by Hezbollah and accused of being a spy for the Americans. He said that Hezbollah told him his "missions" had been fakes -- meant to draw attention away from the group's real operations.

He said he was held and interrogated for months before he eventually made the false confession that he was a CIA agent and had been paid $500,000. (A spokesperson for the CIA referred questions to the Justice Department and FBI, where representatives declined to comment.) Somehow Debek was freed and made his way back to the U.S., where the Department of Justice said he lived in Michigan.

The complaints are not clear on how the FBI first learned about the pair or if they are connected. Though they were arrested on the same day, there's no obvious overlap in the narrative in the criminal complaints against them. Both suspects separately are described as having sat down for several interviews with federal agents beginning in 2016 and into 2017.

At some point during that period an attorney for Kourani approached the FBI in hopes of making a deal, the complaint says. Kourani could provide information in exchange for money and help with immigration for one of Kourani's relatives, the attorney said. The complaint says no promises were made, but Kourani kept talking in five more interviews in 2017.

Then, on June 1, Kourani and Debek were arrested. Debek was brought from Michigan to New York's Southern District and both were charged with a litany of terror-related crimes that could send them away for decades. They're being held in New York, according to court documents.

Ex-FBI agent Garrett said that from the fragments in the complaints, it's difficult to explain the FBI's relationship with the suspects or its sudden ignominious end. But he theorized that one or both may have become an informant, who then failed to uphold their end of the bargain by hiding information, providing misinformation or continuing to work for Hezbollah.

As for what happens next, the cases against each suspect are winding their way through the slow summer court season. In the meantime, it's always possible a plea deal could be made, which could mean an intelligence windfall for Hezbollah activities in the U.S. if Debek and Kourani are who the Justice Department says they are.

"These people would have a lot of information that the law enforcement and intelligence agencies would like to know," former FBI agent Garrett said. "My guess is there's a number of those folks floating around that [law enforcement] knows about, and some that we don't know about."

Primary Source: Criminal Complaint Against Samer El Debek [PDF]

Primary Source: Criminal Complaint Against Ali Kourani [PDF]

[Ed. Note: This report was updated Aug. 8, 2017 to include links to DOJ arrest announcement and criminal complaints. This report was updated Oct. 10, 2017 to add an "allegedly" where one was missing.]