By Kathryn Westcott
Imad Fayez Mughniyeh was a secretive figure, who for decades managed to elude attempts by US spies and special forces to capture or kill him.
Mughniyeh reputedly planned the hijacking of a TWA plane in 1985
He was wanted by the United States and Interpol in connection with bloody attacks around the world.
Some US officials dubbed him the "faceless terrorist", because few outside his closest circle knew for sure what he looked like.
The FBI's picture of him on its most wanted list was 20 years out of date, and he is believed to have undergone plastic surgery to dramatically alter his appearance since then.
Mughniyeh has been in hiding since the end of the 1980s. Until Osama Bin Laden appeared on the radar of US intelligence, he was one of the world's most hunted men.
Before the 11 September attacks on the US, he was alleged to have been involved in the killings of more Americans than anyone else in the world.
In 1983, America was given a wake-up call to suicide terror on a massive scale. Mughniyeh was said to have been the mastermind of that attack - the bombing of the US marine barracks in Lebanon, which killed 241 people.
He was also the prime suspect in two bombings of America's embassy in the Lebanese capital and is believed to have been behind a wave of Western hostage-taking in Lebanon in the 1980s.
In 1985, Mughniyeh was identified as the man behind the brutal hijacking of a jet bound for Rome. Over 17 days, TWA pilot John Testrake was forced to criss-cross the Mediterranean with his 153 passengers and crew, from Beirut to Algiers and back again.
The plane finally came to rest in Beirut. The hijackers threatened to kill the passengers unless hundreds of Lebanese were released from Israeli prisons.
A US navy diver was tortured then killed, his body thrown onto the tarmac.
Mughniyeh is also suspected of having a hand in the bombing of the Israeli embassy in Argentina in 1992, in which 29 people were killed.
Not much is known about his early years, other than that he was born to a prominent Shia religious family in southern Lebanon in 1962. The family then moved to the southern suburbs of Beirut.
According to some reports, he worked with Yasser Arafat's PLO - which set up in Lebanon in the early 1980s - before co-founding Islamic Jihad, a group closely connected with Hezbollah.
He went on to become one of Hezbollah's top leaders.
US intelligence officials have described Mughniyeh as one of the most intelligent and capable operatives they have ever come across.
Robert Baer, who apparently hunted Mughniyeh for years as a CIA officer, once said that Mughniyeh rarely showed himself outside his inner circle. He said he was believed to hide out in both Lebanon and Iran.
"Mughniyeh is probably the most intelligent, most capable operative we've ever run across," Mr Baer told a CBS's 60 Minutes programme a few years ago.
"He enters by one door, exits by another, changes his cars daily, never makes appointments on a telephone, never is predictable... He only uses people that are related to him that he can trust. He is the master terrorist, the grail we are after since 1983."
Mughniyeh represented the secret dimension of Hezbollah
According to Dr Magnus Ranstorp, a specialist in Hezbollah and a terrorism expert at the Swedish National Defence College in Stockholm, as Hezbollah began to emerge as a major political party, Mughniyeh "disappeared into the shadows".
"He was someone who was only brought out for special missions," Dr Ranstorp told the BBC News website.
"He always kept a low profile. He had one foot in Hezbollah and one foot in elements of the Iranian intelligence."
Dr Ranstorp said that from the late-1990s, Mughniyeh had played a key role in creating a link between Hezbollah and the Palestinian militant Islamist organisation Hamas.
He said that his death would be a blow to Lebanon's powerful military organisation of Shia Muslims.
"When US senators latterly talked about Hezbollah's 'A-team', it was Mughniyeh and his closest associates that they were talking about," he said.
Mr Ranstorp, who has interviewed members of Hezbollah many times, said he was very much the secret dimension - or the dark side - of Hezbollah and that it was an unwritten rule never to talk about him.
"Just mentioning his name was considered dangerous," he said.
In Beirut's southern suburbs, Mughniyeh will be remembered by many as a hero.
Senior cleric Sheikh Mohammad Hussein Fadlallah said after hearing of his death that "the resistance has lost one of its pillars".