WASHINGTON (REUTERS) - The US-British missile strike believed to have killed "Jihadi John" came together at lightning speed, but was months in preparation.
Shortly before midnight on Thursday, two US MQ-9 Reaper drones and one British MQ-9 cruised above Raqqa, the Syrian heart of the Islamic State's self-declared caliphate that stretches deep into Iraq, US officials said.
The aircraft's controllers monitored two people who had entered a car.
One, they were certain, was Mohammed Emwazi, the British computer programming graduate who catapulted to infamy in August 2014 when he presented the beheading of American journalist James Foley, the first of several grisly videos in which he presided over the beheadings of foreign hostages.
Brandishing a knife, dressed head-to-toe in black, and speaking with a London accent, Emwazi became known as "Jihadi John", the most potent symbol of the group's brutality and a high-value target for US and British intelligence agencies.
US officials said the US and British military operation to kill Emwazi had been in the works well before the drones finally unleashed Hellfire missiles on Thursday night.
US and British agencies had tracked the Islamic State propagandist and alleged executioner for months before delivering information on his movements and location to the US military, officials said.
In the days leading up to the strike, Emwazi had been moving around Raqqa, visiting his wife's residence and an Islamic State media operations cell, said one US official, speaking on condition of anonymity.
The subsequent US and British operation unfolded quickly.
Two missiles destroyed the car targeted in the strike.
The United States expressed growing optimism on Friday that Emwazi was dead but cautioned that a formal determination would take time.
"We're 100 per cent sure the guy we hit is dead. We are reasonably sure the dead guy is Jihadi John," said one US official, speaking on condition of anonymity.
The strike illustrates an apparent improvement in Western intelligence gathering over the past year or more in a rugged region where reliable on-the-ground information is scarce and where the United States has struggled to infiltrate the extremist group.
Use of American spy satellites, eavesdropping sensors and drones has been expanded over the past year, US officials and a former official said. Some monitoring resources were moved to Syria from Afghanistan.
However, finding and cultivating informants on the ground - so-called human intelligence - would have been critical to the kind of operation which led to the strike on Jihadi John, the former US official said.
"They need to have someone on the ground," the person said.
The attack on Emwazi follows a series of strikes by the United States and Britain against other relatively well known British recruits to the Islamic State in Iraq and Syria (ISIS) movement.
In August, a man from Birmingham regarded as one of ISIS' top computer experts, Junaid Hussain, was killed in a US drone strike.
Around the same time, two other British ISIS recruits, Reyyad Khan and Ruhul Amin, were killed by drone strikes launched by British forces, a European government source confirmed.
The White House said in August that ISIS' second-in-command was killed in a US air strike in Iraq.
MULTIPLE AIR STRIKES
An activist group called Raqqa is Being Slaughtered Silently said Emwazi was killed close to what is known as the Clock Roundabout, several blocks away from Islamic State's main headquarters and the Islamic Court building.
It said the location was one where ISIS carries out public executions.
The activist group said on its Twitter feed that Emwazi was killed at 11.40pm and that there were at least 14 more air trikes around the city between 11.51pm and midnight on Thursday.
ISIS fighters cordoned off the area, it added.
Emwazi participated in videos showing the killings of US journalists Foley and Steven Sotloff, US aid worker Peter Kassig, British aid workers David Haines and Alan Henning, Japanese journalist Kenji Goto, and other hostages.
Kassig, from Indiana, was also known as Abdul-Rahman, a name he took after converting to Islam in captivity.
Emwazi used the videos to threaten the West, admonish its Arab allies and taunt US President Barack Obama and British Prime Minister David Cameron in front of hostages kneeling in orange jump suits.
It was not immediately clear which British agencies led the hunt for Jihadi John, but in the past both Britain's domestic intelligence service, known as MI-5, and its foreign intelligence service, known as MI-6 have been involved with the CIA in operations against Al-Qaeda and its affiliates.
Britain said it had a crucial role in the operation.
"We have been working, with the United States, literally around the clock to track him down. This was a combined effort. And the contribution of both our countries was essential," Cameron said.