Last summer a prominent Kabul politician, who founded a small Sufi political party that supports monarchial rule, received a letter inviting him to join the ISIS branch in Afghanistan.

"This is all part of their activity," the 65-year-old Sayed Ishaq Gailani, who founded the National Solidarity Movement of Afghanistan, told Fox News, shortly before Wednesday’s horrific car bomb explosion in the capital that killed at least 80 people. "They study people. They find out who has a good name. Then they reach out."

Gailani is not only a known figure in elite Afghan circles, but his family members are hereditary leaders of a distinguished Sufi sect, the Qadiriya. Then eight months ago that ISIS letter was followed by the then-leader of the terrorist group, Abdul Hasib, freely venturing into Kabul to have lunch with Gailani.

"He was well-educated, always mentioning the problems in Afghanistan -- the lack of jobs, education, the presence of foreign troops," he recalled.

Ultimately, Gailani, a former mujahedeen who fought the Soviets, declined the persistent invitation but got an insight into the group's shrewd recruitment strategy.

Hasib was slain this year in late April, in a targeted operation that also claimed the lives of two U.S. Army Rangers. The current reigning leader is not known. ISIS in Afghanistan first announced its establishment in January 2015, calling themselves ISIS-K, a reference to the historical name in the Khorasan province.

According to an official at the National Directorate of Security (NDS), Afghanistan's primary intelligence agency for both foreign and domestic affairs, the group has since recruited mostly through technological means, from private Facebook groups and encrypted apps like Telegram, as well as confiscating villagers’ land so they would have no livelihood and thus be forced to work for the brutal groups just to feed their families.

ISIS-K reached its membership and territorial peak in the summer of 2015, but following a concentrated military campaign by Afghan and NATO forces to eliminate the group in its entirely, it has been falling fast. Their numbers now are estimated to be around 1,000, down from a high of about 3,000 but still filled with fighters from Afghanistan, Pakistan, Uzbekistan, Tajikistan, Central Asia, China and beyond.

"Afghanistan is like a salad," Gailani quipped. "Everyone is here."

Commanding Officer Col. Mohammad Nader of the Afghan Border Police also told Fox News that he discovered bodies of several ISIS fighters in the Ziback district of the far northeastern Badakshan province with identification cards belonging to Iraq, the birthplace of the brutal jihadist outfit.

There remains a concern that as ISIS dwindles rapidly in Iraq and Syria, escapees or even leadership could continue the fight and regroup in other countries such as Afghanistan. And even though numbers are low now, they may creep higher come summer.

"As the weather gets warmer and the snow on the mountain melts, the roads will be much clearer and travel into Afghanistan for terrorists much easier," Nader said.

Furthermore, one high-ranking NDS official, who spoke on the condition of anonymity, told Fox News that around 85 percent of ISIS members in Nangarhar have come from Pakistan’s mountainous Waziristan region, and many of its members belong to the Orakzai Pashtun tribe in Pakistan's northwestern tribal areas. These fighters had convened mostly in Afghan’s Achin district, which is where U.S. forces dropped the MOAB last month.

The ISIS indoctrination process, said the official, continues to run rampant in local Waziristan madrassas, where ISIS leaders devote "enormous attention" to radicalizing children as young as 10.

"The young boys are locked in compounds for weeks -- no windows. They are made to draw gardens of naked women. They are taught to believe in paradise and that their country is invaded by infidels," the source said. "The whole process is to make them hate what they are and what they have and give them a cause to die for."

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And while the issue of ISIS slavery in Iraq and Syria has generated massive attention over the past few years -- having barbarically invaded Iraq's Sinjar Mountain and captured thousands of Yazidi girls -- multiple intelligence officials said ISIS-K in Nangarhar province practices keeps female slaves there, too. Yet given the conservative and taboo nature of the crime in much of the country, it has generated little media or mainstream attention.

"ISIS is raping women here as well. They have created a narrative that divorced women in the areas they control must marry an ISIS fighter and women are being kidnapped," an NDS official explained.

Leaders are said to have affixed white flags to the doors of homes that have families with young unmarried girls and black flags on homes in which widowed or divorced women live, instilling a sense of fear in the population that the females inside could be taken at any time.

And despite ISIS-K's dwindling numbers on the battlefield, the presence of sleeper cells -- especially centered around Kabul -- remain a given. On a daily basis, Afghan security officials foil attack plots and dismantle operational networks.

"ISIS is getting a lot of help from local, organized criminal gangs who just want the money," former Afghan Army Chief of Staff Qadam Shah Shahim told Fox News. Shahim recently resigned along with the minister of defense following the deadly Mazar-e-Sharif Army base attack.

ISIS-K operatives have claimed responsibility for a rash of fatal attacks in some of the most secure places in the city, from an attack on Canadian Embassy guards to an assault on a major military hospital to a suicide bombing during an ethnic minority protest. In fact, these three tragedies alone left 150 dead and more than 300 wounded.

Shahim also pointed out that the group does have a remarkable ability to relocate and regroup quickly. He said that talk of dropping the MOAB began six months ago when ISIS had a foothold in Tora Bora, a mountainous region in eastern Afghanistan. But the process was delayed as Afghan officials wanted to first understand the side effects of such a bombing and its environmental impact. When the green light was given, the leadership had already moved to Achin and the planning had to start over.

Moreover, there is apprehension the Taliban and ISIS could at some point merge or at least cooperate with each other.

"This would be very expensive for the international community," cautioned Gailani. "The Taliban is in a bad financial situation, whereas ISIS receives a lot of foreign funding -- as well as food, clothes, ammunition, which encourages people to join them. A lot can happen in a few months."

He said that while the Taliban has much greater numbers, ISIS has a much greater weapons arsenal, including captured Afghan government tanks and brand-new Jeeps and pickup trucks. Shahim, too, noted that while Taliban salaries are unpredictable and are usually in Pakistani rupees, ISIS still manages to pay its high-ranking members in euros the equivalent of $670 to $900 a month.

"And unlike the Taliban, they are not hassling for food and civilian support," Shahim said.

As it stands, the Taliban and ISIS typically fight each other in the eastern provinces and also such southern provinces as Nangahar, but they cooperate in northern areas like Badakshan and Kunduz against government forces. Nader noted that Taliban leaders even provide safe passage for injured ISIS fighters to travel back to Pakistan for medical treatment.

The U.S.-led focus on ISIS stems from the analysis that the Taliban, while a large and vicious force, is a more regional threat, whereas ISIS has more global goals to spread its "caliphate." Yet Afghan officials repeatedly emphasize that ISIS will never be properly eliminated without the squashing of the Taliban, which many refer to as the ideological umbrella to which all terrorists globally now operate.

"The ideology of the two is the same, the level of readiness to do whatever it takes is the same," Dr. Abdullah Abdullah, chief executive officer of Afghanistan, told Fox News. "The interests in the immediate term differ, but at the same time both are against the state institution and want to replace the governance with their own system."

And, on a parting note, Abdullah offered an analogy.

"Remember," he cautioned. "Usama bin Laden also called himself a Talib."