London (CNN) -- The surprise arrival Wednesday of a tall, gray-haired man at a small airport outside of London raised eyebrows -- and it also raised hopes of a breakthrough on many fronts.
The man on the plane was Moussa Koussa, Libya's foreign minister and former intelligence chief, and he was defecting from the regime of Moammar Gadhafi, the highest-ranking official yet to do so.
Koussa was a stalwart defender of the government as recently as a month ago. But in recent weeks, his demeanor has visibly changed. At one recent media briefing, he kept his head down as he read a statement and left early.
He did not tell the Libyan government he was planning to quit before he arrived in Britain, Libyan government spokesman Mousa Ibrahim said Thursday.
But Ibrahim downplayed the defection itself, saying Moussa was an old man in poor health who had not been able to handle the pressure of his job.
"We gave him permission to leave," Ibrahim said, less than 24 hours after Libyan government denials that Koussa had defected, and insisted he was coming back.
The British Foreign Office announced late Wednesday that Koussa had resigned and come willingly to the United Kingdom.
There's debate about whether his departure from Tripoli will weaken Gadhafi, with some saying it will be a signal to other doubters around Libya's leader that it's time to jump ship.
"It could potentially have a devastating impact on morale within the Gadhafi regime," said Noman Benotman, a former Libyan militant now with the Quilliam Foundation, a counter-extremism think tank.
"Strategically, this move attacks the center of gravity within the regime," Benotman said. "I hope other senior figures within the regime now realize that they too need to be part of the solution and not remain part of the problem."
But others say that Koussa was never in Gadhafi's innermost circle and that his departure is a significant blow but not a critical loss to the regime.
Whether his defection precipitates further crumbling of the government in Tripoli or not, Koussa may be able to shed light on Libya's exact role in events like the Lockerbie bombing in 1988 and the killing of a British policewoman in 1984.
"Let's remember, Moussa Koussa is the single most important Libyan official who was responsible for the intelligence service, the planning and execution of (the bombing of) Pan Am 103," CNN national security contributor Frances Fragos Townsend said.
He was also a key player in Libya's decision to give up its weapons of mass destruction program, said Townsend, who was a homeland security adviser in the George W. Bush administration.
"He was the single individual who had to approve or disapprove my going forward to meet with Gadhafi. And so he really does have secrets to tell," she said, adding that Libya's leader confided in Moussa and relied on him.
British intelligence will be questioning him and passing information to Americans, she anticipated.
And she's certain he arranged everything carefully before he boarded the plane for Britain.
"His finances, his immunity from prosecution and his freedom from extradition process -- he would have worked that all before he left," she said.
British Foreign Secretary William Hague said Wednesday that Koussa had not been offered any immunity.
On Thursday, Hague reiterated calls for senior members of the Gadhafi government to step down. Koussa's defection provides evidence "that Gadhafi's regime ... is fragmented, under pressure and crumbling from within," he said.
Koussa is voluntarily speaking with officials in the UK, he said.
The International Criminal Court, which has opened an unprecedented investigation into possible crimes against humanity during the ongoing Libya civil war, said Thursday that Koussa is not a suspect.
"We have no individuals under investigation, and we will see where the evidence leads us," ICC prosecution spokeswoman Florence Olara said.
She said she did not know whether Koussa had been in touch with prosecutors seeking immunity or offering testimony.
But Scottish prosecutors want to question him about the Lockerbie bombing, the government there said Thursday.
Scottish officials said they have informed the Foreign Office in London that they want to speak to him.
"The investigation into the Lockerbie bombing remains open and we will pursue all relevant lines of inquiry," the government said in a statement.
The only person convicted in connection with the bombing, Abdelbeset Ali Mohmed al Megrahi, was freed from prison in Scotland in 2009 and sent home to Libya on the grounds that he had terminal cancer.
CNN's Nic Robertson, Anderson Cooper, Ben Wedeman, Richard Allen Greene and Carol Jordan contributed to this report.