The execution of American journalist James Foley at the hands of Islamic militants, gruesomely displayed in a video that surfaced this week, was a palpable reminder of the shadowy business of kidnappings in the Middle East.

For years, extremist groups like the Islamic State, which claimed responsibility for Foley's murder, have plied this grisly trade. In the process, they have earned millions of dollars in ransoms, further fueling the radicals' rise.

On Wednesday, The New York Times reported that before killing Foley, the Islamic State had tried to extract a multimillion dollar ransom for him.

According to the Committee to Protect Journarlists, at least 39 journalists are currently missing — 20 of them in Syria — but reporters aren't the only victims. Islamic State militants have made "millions and millions" of dollars in ransoms for locals as well as foreigners, according to Danielle Pletka, a former staff member of the Senate Committee on Foreign Relations.

An investigation earlier this year by the Times revealed that al-Qaeda and affiliated groups had earned at least $125 million from kidnappings since 2008. And there are troubling indications that this is a rapidly growing business — the paper reported that more than half that amount was paid just last year.

Fighters of the Islamic State parade in a commandeered Iraqi security forces vehicle in Mosul, Iraq, on June 23. Image: AP Photo/Associated Press

But the Islamic State — which was affiliated with al-Qaeda until al-Qaeda disassociated itself from the group, saying the radicals were too brutal — may be less dependent on ransoms than other extremist organizations, according to Michael Noonan, the national security program director at the Foreign Policy Research Institute.

The group reportedly stole around $425 million from the central bank in Mosul when capturing the northern Iraqi city in June. The loot, combined with significant donations from supporters of the radical cause in the Gulf, has made kidnappings a less vital source of income.

Malian soldiers, working with French forces, battled radical Islamic rebels in Gao, Mali, in February. Image: AP Photo/Associated Press

The Islamic State has abducted hundreds of civilians from territories they have assaulted. They've slaughtered many of them in order to intimidate their enemies, and they've captured women to force them into marriage or sexual slavery.

Sometimes, as was the case with Foley, hostages are killed in order to communicate a message.

The text "A Message to America" appears onscreen midway through the video, just before Foley renounces the United States, presumably under duress.

At the end of the video, Foley's killer appears again with another American journalist, Steven Joel Sotloff, and says that Sotloff's life depends on President Barack Obama's "next decision."

As part of the group's overall propaganda effort, the video was intended to "instill fear" and show the militants' "level of commitment and zeal," Noonan said.

As such, it aids the extremists' recruiting efforts, according to Pletka, who is now a senior vice president at the American Enterprise Institute, a conservative think tank. She added that propaganda can be especially helpful when the U.S. responds, as Obama did when he addressed the video.

"They want to show their effectiveness," Pletka said. "They want to show that the president of the United States listens to them."