Hostages in the Bangladesh cafe who were unable to recite verses from the Qu'ran were reportedly tortured and killed, while those who could were spared.
The 10-hour siege at the Holey Artisan Bakery in the country's capital Dhaka, ended with 20 hostages and two Bangladeshi police officers dead. Six of the attackers were also killed.
The gunmen asked everyone inside to recite from the Qu'ran. Those who recited were spared. The gunmen even gave them meals last night.
Those who could not recite from the Qu'ran "were tortured", Mr Karim added.
Authorities are currently interrogating one of the attackers who was captured by commandos during the rescue.
The White House has confirmed that a US citizen was among the 20 hostages killed in an attack on a cafe in the Bangladeshi capital of Dhaka.
So-called Islamic State have claimed responsibility for the attack.
So-called Islamic State has claimed responsibility for the attack on a Bangladesh cafe, saying it was targeting the citizens of "crusader countries".
The statement was circulated by supporters of the extremist group following the attack in the country's capital, Dhaka, which left 20 dead.
It is not yet clear if the leadership of so-called Islamic State in Iraq and Syria was involved in the planning of the attack. The group has previously claimed attacks carried out by so-called lone wolf attackers with no known connection to the group.
The Islamic State have claimed responsibility for the attack on a cafe in the capital Dhaka, the Associated Press reports.
The group said it targeted foreigners in the attack, reports say.
BREAKING: Islamic State group claims attack in Bangladesh, says it targeted foreigners.
Bangladesh will stand up and fight the "terror threat" that has mushroomed in its backyard, the country's prime minister has said.
In a live television broadcast, Sheikh Hasina said the country stood "committed and determined" to uphold its freedom, and announced two days of national mourning in the wake of a deadly Islamist militant attack on an upmarket restaurant in Bangladesh's capital Dhaka.
Seven Japanese are among the dead in the Bangladesh cafe siege.
Japanese Chief Cabinet Secretary YoshihideSuga confirmed that five men and two women had been killed.
Nine Italians were among 20 foreign nationals killed in Friday's attack by suspected Islamist militants at a cafe in Bangladesh, Italy's foreign minister has said.
Security forces stormed the restaurant in Dhaka and ended a 12-hour standoff on Saturday, killing six gunmen.
"We have identified nine (Italians) killed, there is another person who is missing and could be hiding himself or could be among wounded people ... we are looking for him," Foreign Minister Paolo Gentiloni told reporters.
Italian Prime Minister Matteo Renzi has condemned the attack on a Dhaka cafe as he confirmed that Italians had been killed.
Renzi said he would not disclose any more details until families of the dead are notified.
All 20 hostages killed in Dhaka cafe siege were foreign, the Bangladesh army has said.
Condemning the deadly attack by suspected Islamist militants, Bangladesh's Prime Minister Sheikh Hasina said:
It was an extremely heinous act.
Anyone who believes in religion cannot do such act.
They do not have any religion, their only religion is terrorism.