The Islamic State, the radical group which has cut a bloody path through Syria and Iraq in recent months, slaying civilians and security forces, and executing the American journalist James Foley on camera, has even more followers than previously believed, according to Iraqi security sources.

While western diplomats and other observers have estimated the number of militants number between 20,000 to 50,000, Hisham al-Hashimi, a security expert in Baghdad, said that the total membership of the Islamic State in Iraq and Syria may now be closer to 100,000 members.

A convoy of vehicles and fighters from the Islamic State of Iraq and the Levant roll through Iraq's Anbar Province on Jan. 7. Image: Associated Press via militant website/Associated Press

If true, the numbers suggest that the militant organization is growing by leaps and bounds as it conquers territory in the region.

According to Hashimi, a key event in terms of adding more people to its rank was the group’s capture of Mosul in June. The northern Iraqi city is a Sunni stronghold, home to many former officers in Saddam Hussein’s army who have opposed the Shiite-dominated government in Baghdad. And while some people were forced to enlist, others seem to have voluntarily taken up arms.

As a researcher who occasionally gives advice to the Iraqi security forces, Hashimi has had access to hundreds of memory drives recovered by Iraqi security forces at the home of the second-in-command of the Islamic state and other Iraqi intelligence as well as researchers on the ground in Iraq.

“The Islamic State didn't come from nowhere,” said Hashimi who has been studying the group for years. “It is an extension of groups that existed before — historically and ideologically.”

Hashimi’s estimate was seconded by Ibrahim al-Sumaidaei, a political analyst, currently based in Amman, Jordan.

“The Islamic State’s members have multiplied in a very dangerous way,” said Sumaidaei, a former intelligence officer. “Having plenty of arms and funding has made the Islamic State swallow the fighters of the other Sunni insurgent groups.” Sumaidaei added that many of the members may not be “active duty.”

In this June 23, 2014 photo, Islamic State of Iraq and the Levant (ISIL) fighters parade in a commandeered Iraqi security forces armored vehicle down a main road at the northern city of Mosul, Iraq. Image: STR/Associated Press

In addition to foot soldiers, the Islamic State has also attracted a number of former high level army officers from Saddam Hussein's Baathist regime, said Hashimi. (Many Baathists fought American troops after the 2003 U.S. invasion of Iraq.)

“Most of those who joined — and I know them personally — are either former army officers or their sons,” said Salem Aljomaily, a security expert and a former intelligence officer. In terms of how the new recruits are organized, Aljomaily said that “the high ranking officers are the military planners, and their sons and the younger officers are being used as fighters.”

The Islamic State is run by Abu Bakr al-Baghdadi, a Sunni radical who once served time in an American military jail in Iraq. The forces in both Syria and Iraq are run by former high-ranking Baathist officers, according to Hashimi.

A screenshot from a video released on July 5 shows alleged Islamic State of Iraq and the Levant (ISIL) leader Abu Bakr al-Baghdadi preaching at a mosque in Mosul, Iraq. Image: Al-Furqan Media/Anadolu Agency/Getty Images

The organization is divided roughly evenly between Syria and Iraq when it comes to its membership. In terms of organization, Hashimi estimates that, in Iraq, about half the members are fighters; the other half are involved in logistics – including administration and transportation – and business: controlling the movement and sale of illicit oil, among other things. In Syria, he said, the organizational breakdown was roughly the same.

Harith Hassan, a fellow at the Radcliffe Institute at Harvard University, said that “capturing Mosul made the group appear supremely attractive” because it now appears as the most victorious of all the radical Sunni groups. In some ways, Hassan said, the Islamic State “has replaced al-Qaeda as the most prominent Jihadi group.”

According to Hashimi, the Islamic State now numbers at least 7,000 foreign fighters, about half from North Africa. Many are young men who have been radicalized in the aftermath of the Arab Spring in places like Tunisia, Libya and Algeria. There are also several hundred Chechen and east European recruits, and about 2,000 people from northern Europe, including about 450 radicals from Britain, according to Hashimi.

In part, the ranks of the Islamic State has swelled in response to the rise of Shiite militias and sectarian fear. “For the residents of the Sunni cities, the Islamic State is the lesser of two evils,” said Sumaidaei.

And rather than curtail the ambitions of the radical group, American air strikes have instead helped recruitment efforts, said Hashimi. “Baghdadi carries now the flag of the Jihadi against the crusader.”

Riyadh Mohammed is a multimedia journalist who has worked for the New York Times and the Los Angeles Times and Way Press International.