The four French hostages, seized by Al-Qaeda in the Islamic Maghreb (AQIM) in northern Niger in 2010, returned to France on Wednesday after more than three years in captivity.
French officials have repeatedly denied that a ransom was paid for their release, but sources in Niger and France have told AFP and other media that more than 20 million euros ($28 million) was paid to AQIM and intermediaries.
Quoting a well-placed French source, newspaper Le Monde said the payment came directly from "secret funds allocated to the intelligence services".
Experts said there was little doubt a payment was made.
"Governments and companies pay in almost every case," a former senior French intelligence official told AFP, speaking on condition of anonymity.
"There is always a ransom or an exchange of some sort: money, the release of prisoners, arms deliveries."
The US Treasury undersecretary for terrorism and financial intelligence, David Cohen, said last year that "terrorist organisations" had raised more than $120 million in ransom payments in the previous eight years.
He told an audience at Chatham House that the amounts of ransom demands were going up and that Europeans were being mainly targeted because of the belief their governments would pay.
"AQIM, the Al-Qaeda affiliate that has likely profited most from kidnapping for ransom, has collected tens of millions of dollars through KFR (kidnapping for ransom) operations since 2008," he said.
"It has become increasingly clear that AQIM is using revenues from kidnapping for ransom to expand its reach and influence."
As well as the French, the Spanish and Italians have been singled out for allegedly paying ransoms, including a reported 15 million euro payment last year to free two Spaniards and an Italian held in Mali.
"There is an established precedent of ransoms being paid to kidnappers... This primarily involves European governments," said Geoff Porter, the head of New York-based North Africa Risk Consulting.
"If you pay ransoms to kidnappers, then there's a potential industry in hostage-taking," he said.
The United States and Britain have repeatedly raised concerns about such payments, with Britain using its presidency of the G8 this year to push for a ban on ransoms.
G8 leaders gathered in Northern Ireland in June signed a deal to "unequivocally reject the payment of ransoms to terrorists" but did not impose a formal ban.
British officials said at the time that Al-Qaeda-linked groups around the world had made $70 million from mainly Western hostages over the last two years.
A former US ambassador to Mali, Vicki Huddleston, said this year that European governments were "helping AQIM to grow strong, buy weapons and recruit" with ransom payments.
"The US gets very angry about this... Some of the European governments have been seen as softer on this issue," said Raffaello Pantucci, a senior research fellow in counter-terrorism at the Royal United Services Institute.
"The long-term effects of this are that, first, the groups will probably do it again and second, it gives the organisation money... to swell its coffers," he said.
Pantucci said that in many cases ransom payments were made not by governments but by employers and family members, making them more difficult to oversee.
"Governments don't control all the reins when it comes to this issue," he said.
And experts said it was easy understand why many governments want to keep all options on the table when it comes to rescuing hostages.
"At the end of the day, it's awfully difficult to look into the eyes of family members of hostages and say that as a matter of principle we are going to let them die," Porter said