Libya's Foreign Minister Moussa Koussa holds a news conference in Tripoli in this March, 18, 2011 file photo.Zohra Bensemra / Reuters
He was Colonel Moammar Gadhafi's right-hand man, a onetime pariah accused of masterminding the Lockerbie bombing, and Libya's longest-serving spy chief.
Now, after defecting to Britain, Moussa Koussa, the Libyan foreign minister, could prove to be the West's greatest asset in dislodging Col. Gadhafi from power and shedding light on what role his regime may have played in the downing of Pan Am Flight 103.
The 61-year-old left his wife and some of his children behind in Tripoli when he crossed the border into Tunisia under the guise of seeking medical treatment. From there he contacted the British embassy and hired a private jet to fly him to London. When he landed he was debriefed by British intelligence, although it remains unclear whether he will be granted immunity from prosecution related to his alleged crimes.
His arrival in Britain sparked mixed reaction from families of the Lockerbie victims.
Jim Swire, whose daughter, Flora, was killed in the attack, and who has spoken with Mr. Koussa several times, calling him "the scariest man" he has ever met, said his defection could be the key to unlocking the truth about the bombing.
"Within the Libyan regime [Mr. Koussa]is in the best position of anyone other than Gadhafi himself to tell us what the regime knows or did," Mr. Swire said. "He would be a peerless source of information."
Mr. Koussa's defection is the strongest sign yet that Col. Gadhafi's regime is vulnerable, British Foreign Secretary William Hague said.
"His resignation shows that Gadhafi's regime, which has already seen significant defections to the opposition, is fragmented, under pressure and crumbling from within. Gadhafi must be asking himself who will be the next to abandon him," Mr. Hague said.
His defection could also signify a deeper shift within the Gadhafi regime around seeking an exit strategy.
In the wake of Mr. Koussa's defection the Guardian newspaper reported that Mohammed Ismail, a senior aide to Col. Gadhafi's son Saif al-Islam, had visited London in recent days for confidential talks with British officials - a further sign, analysts said, that the Libyan leader may be trying to negotiate a way out.
Turning on the regime he spent a lifetime serving is the latest twist in the complicated life of Mr. Koussa, whose deeply lined face framed by a shock of white hair was never far from Col. Gadhafi's side.
His relationship with the Libyan leader grew out of an academic fascination with him, dating back to his days as a graduate student at the University of Michigan in 1978.
Mr. Koussa wrote his master's thesis on "The Leader," interviewing him twice and meticulously questioning his friends and family members to paint an intricate portrait of his subject. As it turned out, Mr. Koussa was the one being unknowingly interviewed. Col. Ghadafi was impressed by the young scholar and persuaded him to trade academics for politics, joining his inner circle.
The Libyan leader appointed his protégé ambassador to Britain in 1980. A year later he granted an interview to The Times of London, where he shockingly revealed he had ordered the killing of Libyan dissidents on British soil.
"The revolutionary committees have decided last night to kill two more people in the United Kingdom. I approve of this," Mr. Koussa told The Times.
The statement earned him a one-way ticket back to Tripoli, where he remained one of Col. Gadhafi's most trusted aides.
A U.S. embassy cable published by WikiLeaks described him as "the rare Libyan official who embodies a combination of intellectual acumen, operational ability and political weight."
In Libya, he was accused of orchestrating terror and being the possible architect of the 1988 bombing of Pan Am Flight 103 that killed 270 people. He was also accused in the plot that resulted in the downing of a French airliner in central Africa the next year, where 170 people died. Mr. Koussa denies any involvement in either incident. He was appointed head of Libya's External Security Organization in 1994, a time when Col. Gadhafi's regime was constantly being linked to terrorist activity.
In the aftermath of Sept. 11, Mr. Koussa emerged as a key player in sharing intelligence about al-Qaeda with British security forces.
Mr. Koussa leveraged his power to forge lasting links with British and U.S. intelligence services, helping negotiate the surrender of Libya's weapons of mass destruction program and funnelling intelligence around the case of Pakistani nuclear scientist A.Q. Khan.
He also played a central role in negotiating the release of Abdel Baset al-Megrahi, the Lockerbie bomber whose return to Libya he eventually secured.
Mr. Koussa, who was appointed foreign minister in May of 2009, is apparently in ill health. Some analysts say his relationship with the Gadhafi family has also become strained.
There are unconfirmed reports of a fistfight erupting between Mr. Koussa and one of Mr. Gadhafi's sons. At an international summit in Tripoli last year, Mr. Koussa was seen smoking in the dining hall while the rest of Mr. Gadhafi's inner circle met in a private room, a sign, some said, that the foreign minister was being distanced from the regime.
At one of his last public appearances as Libya's foreign minister, he announced a ceasefire after the United Nations Security Council voted to impose a no-fly zone. Several reports noted that as he spoke, Mr. Koussa's hands were shaking.